Method for producing water absorbent polyacrylic acid (salt) resin powder, and water absorbent polyacrylic acid (salt) resin powder

ABSTRACT

An object of the present invention is to provide a method for producing water absorbent resin powder in which permeability potential (SFC) is improved while a water absorbing rate (FSR) is being kept. The method is a method for producing water absorbent polyacrylic acid (salt) resin powder including the steps of: (i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii) during or after the step of (i), performing gel grinding of a hydrogel crosslinked polymer obtained by the polymerization, wherein the hydrogel crosslinked polymer has resin solid content of 10 wt % to 80 wt %, and the gel grinding is carried out with gel grinding energy (GGE) of 18 [J/g] to 60 [J/g]; (iii) drying a particulate hydrogel crosslinked polymer obtained by the gel grinding, wherein the drying is performed at 150° C. to 250° C.; and (iv) carrying out a surface treatment to the particulate hydrogel crosslinked polymer thus dried.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Section 371 U.S. national stage entry ofpending International Patent Application No. PCT/JP2011/058829,International Filing Date Apr. 7, 2011, which published on Oct. 13, 2011as Publication No. WO 2011/126079, which claims the benefit of JapanesePatent Application No. 2010-088993,filed Apr. 7, 2010, Japanese PatentApplication No. 2010-179515, filed Aug. 10, 2010, and Japanese PatentApplication No. 2011-031287, filed Feb. 16, 2011, the contents of whichare incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to a method for producing water absorbentpolyacrylic acid (salt) resin powder, and the water absorbentpolyacrylic acid (salt) resin powder. More specifically, the presentinvention relates to a method for producing water absorbent polyacrylicacid (salt) resin powder having excellent water absorbing ability(especially having high permeability potential and water absorbingrate), and the water absorbent polyacrylic acid (salt) resin powder,which water absorbent polyacrylic acid (salt) resin powder is waterabsorbent resin powder for use in sanitary goods such as disposablediaper, sanitary napkins, etc.

BACKGROUND ART

Water absorbent resin (SAP/Super Absorbent Polymer) is a polymer gellingagent which is swellable with water but insoluble with water. The waterabsorbent resin is widely used, mainly disposably, for absorbingproducts such as disposable diapers, sanitary napkins, etc., and furtherfor an agriculture/horticulture water retaining agent, an industrialwaterproofing agent, and the like. For such water absorbent resin, manymonomers and hydrophilic polymers have been proposed as raw materials.Especially, water absorbent polyacrylic acid (salt) resin in whichacrylic acid and/or its salt is used as its monomer is most popular inindustries because of its high water absorbing ability (Non-PatentLiterature 1).

The water absorbent resin is produced via a polymerizing step, a dryingstep, and if necessary, a non-dried matter removing step, a grindingstep, a classifying step, a surface cross-linking step, and/or the like(Patent Literatures 1 to 5, and 50). Meanwhile, the water absorbentresin is required to have many functions (properties) in order to copewith functional sophistication of disposable diapers which are one majorapplication of the water absorbent resin. More specifically, the waterabsorbent resin is required to satisfy many properties such as, not onlya high water absorbing coefficient, but also gel strength, water solublecontent, a water absorbing rate, an absorbency against pressure,permeability potential, particle size distribution, an anti-urineproperty, an anti microbial property, impact resistance (an anti-damageproperty), fluidity, an deodorant property, anti-coloring (degree ofwhiteness), low dustiness, etc. Therefore, many cross-linkingtechniques, additives, modifications in steps in the production, etc.have been proposed.

Among these properties, the permeability potential is considered as amore important factor in association with a recent increase (forexample, 50 wt % or more) in an amount of the water absorbent resin indisposable diapers. Furthermore, methods and techniques for improvingpermeability potential against pressure and permeability potentialwithout pressure, such as SFC (Saline Flow Conductivity, see PatentLiterature 6) or GBP (Gel Bed Permeability, see Patent Literatures 7 to9), have been proposed.

Various combinations of a plurality of parameters (including thepermeability potential) of the properties have been also proposed. Therehave been known a technique for defining impact resistance (FI) (PatentLiterature 10), a technique for defining, for example, a water absorbingrate (FSR/Vortex) (Patent Literature 11), and a technique for definingthe product of liquid diffusivity (SFC) and core absorption quantityafter 60 minutes (DA60) (Patent Literature 12).

As the method for improving the permeability potential such as SFC andGBP, there have been known a technique for adding plaster before orduring polymerization (Patent Literature 13), a technique for addingspacers (Patent Literature 14), a technique for using anitrogen-containing polymer having 5 through 17 [mol/kg] of nitrogenatoms which can be protonated (Patent Literature 15), a technique forusing polyamine, and polyvalent metal ions or polyvalent anions (PatentLiterature 16), a technique for covering, with polyamine, waterabsorbent resin having a pH of less than 6 (Patent Literature 17), and atechnique for using polyammonium carbonate (Patent Literature 18). Inaddition, there have been known a technique for using polyamine havingwater soluble content of not less than 3%, and a technique for defininga suction index (WI) and gel strength (Patent Literatures 19 through21). There have been also known techniques for using polyvalent metalsalt while controlling, during polymerization, methoxyphenol that is apolymerization inhibitor, in order to improve coloring and thepermeability potential (Patent Literatures 22 and 23). Moreover, therehas been known a technique for polishing particles so as to attain ahigh bulk specific gravity (Patent Literature 24).

Moreover, in addition to the permeability potential, the water absorbingrate is also a basic property for the water absorbent resin. As onemethod for improving the water absorbing rate, a technique to increase aspecific surface area in order to attain a greater water absorbing rateis known. More specifically, a technique for controlling to attain fineparticle diameters (Patent Literature 25), techniques for granulatingfine particles with a large surface area (Patent Literatures 26 to 28),a technique for freeze-drying a hydrogel to cause the hydrogel to beporous (Patent Literature 29), techniques for performing granulation andsurface cross-linking of particles simultaneously (Patent Literatures 30to 32), techniques for foaming polymerization (Patent Literatures 33 to48), a technique for post-polymerization foaming and cross-linking(Patent Literature 49), etc. have been proposed.

More specifically, as to the foaming polymerization, the followingtechniques have been known regarding a foaming agent for treating amonomer(s): techniques for using a carbonate (Patent Literatures 33 to40), techniques for using an organic solvent (Patent Literatures 41 and42), techniques for using an inert gas (Patent Literatures 43 to 45),techniques for using an azo compound (Patent Literatures 46 and 47), atechnique for using insoluble inorganic powder (Patent Literature 48),etc.

CITATION LIST Patent Literatures

Patent Literature 1

U.S. Pat. No. 6,576,713 B, Specification

Patent Literature 2

U.S. Pat. No. 6,817,557 B, Specification

Patent Literature 3

U.S. Pat. No. 6,291,636 B, Specification

Patent Literature 4

U.S. Pat. No. 6,641,064 B, Specification

Patent Literature 5

US Patent Application Publication, No. 2008/0287631A, Specification

Patent Literature 6

U.S. Pat. No. 5,562,646 B, Specification

Patent Literature 7

US Patent Application Publication No. 2005/0256469 A, Specification

Patent Literature 8

U.S. Pat. No. 7,169,843 B, Specification

Patent Literature 9

U.S. Pat. No. 7,173,086 B, Specification

Patent Literature 10

U.S. Pat. No. 6,414,214 B, Specification

Patent Literature 11

U.S. Pat. No. 6,849,665 B, Specification

Patent Literature 12

US Patent Application Publication No. 2008/125533 A, Specification

Patent Literature 13

US Patent Application Publication No. 2007/293617 A, Specification

Patent Literature 14

US Patent Application Publication No. 2002/0128618 A, Specification

Patent Literature 15

US Patent Application Publication No. 2005/0245684 A, Specification

Patent Literature 16

PCT International Publication No. 2006/082197 A, Pamphlet

Patent Literature 17

PCT International Publication No. 2006/074816 A, Pamphlet

Patent Literature 18

PCT International Publication No. 2006/082189 A, Pamphlet

Patent Literature 19

PCT International Publication No. 2008/025652 A, Pamphlet

Patent Literature 20

PCT International Publication No. 2008/025656 A, Pamphlet

Patent Literature 21

PCT International Publication No. 2008/025655 A, Pamphlet

Patent Literature 22

PCT International Publication No. 2008/092843 A, Pamphlet

Patent Literature 23

PCT International Publication No. 2008/092842 A, Pamphlet

Patent Literature 24

U.S. Pat. No. 6,562,879 B, Specification

Patent Literature 25

US Patent Application Publication No. 2007/015860 A, Specification

Patent Literature 26

U.S. Pat. No. 5,624,967 B, Specification

Patent Literature 27

PCT International Publication No. 2005/012406 A, Pamphlet

Patent Literature 28

U.S. Pat. No. 5,002,986 B, Specification

Patent Literature 29

U.S. Pat. No. 6,939,914 B, Specification

Patent Literature 30

U.S. Pat. No. 5,124,188 B, Specification

Patent Literature 31

EP Patent No. 0595803 B, Specification

Patent Literature 32

EP Patent No. 0450922 B, Specification

Patent Literature 33

U.S. Pat. No. 5,118,719 B, Specification

Patent Literature 34

U.S. Pat. No. 5,154,713 B, Specification

Patent Literature 35

U.S. Pat. No. 5,314,420 B, Specification

Patent Literature 36

U.S. Pat. No. 5,399,591B, Specification

Patent Literature 37

U.S. Pat. No. 5,451,613 B, Specification

Patent Literature 38

U.S. Pat. No. 5,462,972 B, Specification

Patent Literature 39

PCT International Publication No. 95/02002 A, Pamphlet

Patent Literature 40

PCT International Publication No. 2005/063313 A, Pamphlet

Patent Literature 41

PCT International Publication No. 94/022502 A, Pamphlet

Patent Literature 42

U.S. Pat. No. 4,703,067 B, Specification

Patent Literature 43

PCT International Publication No. 97/017397 A, Pamphlet

Patent Literature 44

PCT International Publication No. 00/052087 A, Pamphlet

Patent Literature 45

U.S. Pat. No. 6,107,358 B, Specification

Patent Literature 46

U.S. Pat. No. 5,856,370 B, Specification

Patent Literature 47

U.S. Pat. No. 5,985,944 B, Specification

Patent Literature 48

PCT International Publication No. 2009/062902 A, Pamphlet

Patent Literature 49

EP Patent No. 1521601B, Specification

Patent Literature 50

Japanese Patent Application Publication, Tokukaihei, No. 11-349687 A

Non-Patent Literatures

Non-Patent Literature 1

Modern Superabsorbent Polymer Technology (1998), (especially, p. 197 to199)

SUMMARY OF INVENTION Technical Problem

In order to improve properties of water absorbent resin, manycross-linking techniques, additives, modifications in steps in theproduction, etc. have been thus proposed. Among these properties,permeability potential and a water absorbing rate are significant asbasic properties of the water absorbent resin, and therefore manytechniques for improving the permeability potential and the waterabsorbing rate have been proposed so far.

Meanwhile, the permeability potential is inversely proportional to thewater absorbing rate. That is, increase in one of the permeabilitypotential and the water absorbing rate causes decrease in the other ofthe permeability potential and the water absorbing rate. Conventionaltechniques improve just one of the permeability potential and the waterabsorbing rate. Therefore, there has been required a technique forimproving the one without decreasing the other, or a technique forimproving both of them.

In order to satisfy such requirement, an object of the present inventionis to provide (i) a method for producing water absorbent resin powder inwhich permeability potential (SFC) and a water absorbing rate (FSR) areattained (particularly, the permeability potential is improved while thewater absorbent rate is being kept), and (ii) the water absorbent resinpowder.

Solution to Problem

In order to attain the object, it was found that it is possible toattain both the permeability potential (SFC) and the water absorbingrate (FSR) (particularly, to improve the permeability potential whilekeeping the water absorbent rate) by changing a shape of water absorbentresin, the shape of the water absorbent resin being changed by drying aparticulate hydrogel crosslinked polymer that are obtained by grinding ahydrogel crosslinked polymer by appropriate shearing stress andcompressive force (grinding with gel grinding energy (GGE) of 18 [J/g]through 60 [J/g] or gel grinding energy (2) (GGE (2)) of 9 [J/g] through40 [J/g], or increasing weight average molecular weight of water solublecontent of the hydrogel crosslinked polymer by 10,000 [Da] through500,000 [Da] in comparison with pre-gel grinding). Based on the finding,the present invention was accomplished.

A method for producing water absorbent resin powder of the presentinvention (first producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization, wherein the hydrogelcrosslinked polymer has resin solid content of 10 wt % to 80 wt %, andthe gel grinding is carried out with gel grinding energy (GGE) of 18[J/g] to 60 [J/g]; (iii) drying a particulate hydrogel crosslinkedpolymer obtained by the gel grinding, wherein the drying is performed at150° C. to 250° C.; and (iv) carrying out a surface treatment to theparticulate hydrogel crosslinked polymer thus dried.

A method for producing water absorbent resin powder of the presentinvention (second producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization, wherein the hydrogelcrosslinked polymer has resin solid content of 10 wt % to 80 wt %, andthe gel grinding is carried out with gel grinding energy (2) (GGE (2))of 9 [J/g] to 40 [J/g]; (iii) drying a particulate hydrogel crosslinkedpolymer obtained by the gel grinding, wherein the drying is performed at150° C. to 250° C.; and (iv) carrying out a surface treatment to theparticulate hydrogel crosslinked polymer thus dried.

A method for producing water absorbent resin powder of the presentinvention (third producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization, wherein the hydrogelcrosslinked polymer has resin solid content of 10 wt % to 80 wt %, sothat weight average molecular weight of water soluble content of thehydrogel crosslinked polymer is increased by 10,000 [Da] to 500,000[Da]; (iii) drying a particulate hydrogel crosslinked polymer obtainedby the gel grinding, wherein the drying is performed at 150° C. to 250°C.; and (iv) carrying out a surface treatment to the particulatehydrogel crosslinked polymer thus dried.

Further, in order to attain the object, it was found that it is possibleto attain both the permeability potential (SFC) and the water absorbingrate (FSR) of the water absorbent resin powder (particularly, to improvethe permeability potential while keeping the water absorbent rate) by(i) drying, under a specific condition, a particulate hydrogelcrosslinked polymer having all properties of specific weight averageparticle diameter, specific logarithmic standard deviation of particlesize distribution, and specific resin solid content, and then (ii)performing a surface treatment to the particulate hydrogel crosslinkedpolymer. Based on the finding, the present invention was accomplished.

A method for producing water absorbent resin powder of the presentinvention (fourth producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization so as to obtain aparticulate hydrogel crosslinked polymer having a weight averageparticle diameter (D50) of 350 μm to 2000 μm and logarithmic standarddeviation (σζ) of particle size distribution of 0.2 to 1.0; (iii) dryingthe particulate hydrogel crosslinked polymer by hot air of 150° C. to250° C. at a velocity of 0.8 [m/s] to 2.5 [m/s] in a direction vertical(up-and-down direction) to the particulate hydrogel crosslinked polymerby use of a through-flow belt drier, the particulate hydrogelcrosslinked polymer to be supplied into the through-flow belt drierhaving resin solid content of 10 wt % to 80 wt %; and (iv) carrying outa surface treatment to the particulate hydrogel crosslinked polymer thusdried.

In other words, a method for producing water absorbent resin powder ofthe present invention (first to fourth producing methods) is a methodfor producing water absorbent polyacrylic acid (salt) resin powder,including the steps of: (i) polymerizing an acrylic acid (salt) monomeraqueous solution; (ii) during or after the step of (i), performing gelgrinding of a hydrogel crosslinked polymer obtained by thepolymerization, wherein the hydrogel crosslinked polymer has resin solidcontent of 10 wt % to 80 wt %; (iii) drying a particulate hydrogelcrosslinked polymer obtained by the gel grinding, wherein the drying isperformed at 150° C. to 250° C.; and (iv) carrying out a surfacetreatment to the particulate hydrogel crosslinked polymer thus dried,the step of (ii) being carried out such that at least one of (1) to (4)is met, where (1) the gel grinding is carried out with gel grindingenergy (GGE) of 18 [J/g] to 60 [J/g]; (2) the gel grinding is carriedout with gel grinding energy (2) (GGE (2)) of 9 [J/g] to 40 [J/g]; (3)weight average molecular weight of water soluble content of the hydrogelcrosslinked polymer is increased by 10,000 [Da] to 500,000 [Da]; and (4)the particulate hydrogel crosslinked polymer obtained by the step of(ii) has a weight average particle diameter (D50) of 350 μm to 2000 μm,and logarithmic standard deviation (σζ) of particle size distribution of0.2 to 1.0.

In a case where the hydrogel crosslinked polymer is subjected to the gelgrinding so that (4) is met, the particulate hydrogel crosslinkedpolymer to be supplied into a through-flow belt drier has resin solidcontent of 10 wt % to 80 wt %, and the through-flow belt drier sends hotair of 150° C. to 250° C. at a velocity of 0.8 [m/s] to 2.5 [m/s] in adirection vertical (up-and-down direction) to the particulate hydrogelcrosslinked polymer.

The gel grinding of the present invention essentially meets at least oneof (1) through (4), preferably two or more, further preferably three ormore, and especially preferably all of them. Further, it is preferablethat not only the particulate hydrogel crosslinked polymer obtained bythe gel grinding of (4) but also the particulate hydrogel crosslinkedpolymer obtained by the gel grinding of (1) to (3) be dried by thethrough-flow belt drier under the drying condition (such as the hot airvelocity). Moreover, it is further preferable that surface crosslinkingbe performed especially by combination use of a covalent bonding surfacecrosslinking agent and an ionic bonding surface crosslinking agent which(later described).

In order to attain the object of the present invention (to attain boththe permeability potential (SFC) and the water absorbing rate (FSR) ofthe water absorbent resin powder (particularly, to improve thepermeability potential while keeping the water absorbent rate)), thewater absorbent polyacrylic acid (salt) resin of the present inventionis water absorbent polyacrylic acid (salt) resin having: 95 wt % or moreof particles whose diameter is not less than 150 μm and less than 850μm; logarithmic standard deviation (σζ) of particle size distribution of0.25 to 0.50; absorption against pressure (AAP) of 20 [g/g] or more; awater absorbing rate (FSR) of 0.30 [g/g/s] or more; and an internal cellratio of 0.1% to 2.5%, which is calculated by the following expression.(Internal cell ratio) [%]={(real density)−(apparent density)}/(realdensity)×100

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, it is possible to produce waterabsorbent resin having high permeability potential (SFC, etc.) and ahigh water absorbing rate (FSR, etc.) by (i) applying appropriateshearing stress and compressive force to a hydrogel crosslinked polymerby at least one gel grinding of the above-described (1) through (4),(ii) drying the hydrogel crosslinked polymer, and (iii) furthersubjecting dried hydrogel crosslinked polymer to a surface treatment.

Further, it is possible to produce water absorbent resin powder havingpermeability potential (SFC) and a water absorbing rate (FSR) higherthan those of water absorbent resin produced by a conventionalproduction method, by (i) drying, under a specific condition, aparticulate hydrogel crosslinked polymer having specific weight averageparticle diameter, specific logarithmic standard deviation of particlesize distribution, and specific resin solid content, the particulatehydrogel crosslinked polymer being obtained by applying appropriateshearing stress and compressive force to a hydrogel crosslinked polymerby gel grinding of (4), etc., and then (ii) subjecting a driedparticulate hydrogel crosslinked polymer to a surface treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically illustrating a configuration of ascrew extruder to be used in a step of grinding a hydrogel crosslinkedpolymer.

FIG. 2 is a cross-sectional view schematically illustrating closed cellsand open cells in water absorbent resin powder.

FIG. 3 is a cross-sectional view schematically illustrating how tofinely grind to less than 45 μm water absorbent resin powder (forexample, containing particles in particle diameter of 850 μm to 150 μmby not less than 95 wt %) in order to perform real density measurementin the present invention.

DESCRIPTION OF EMBODIMENTS

In the following, a method for producing water absorbent polyacrylicacid (salt) resin powder and the water absorbent polyacrylic acid (salt)resin powder according to the present invention are described in detail.It should be noted that the scope of the present invention is notlimited to the description and the present invention can be embodiedwith modifications other than the following exemplary embodiments butnot departing from the gist of the present invention. More specifically,the present invention shall not be construed as being limited to thefollowing embodiments, and may be modified in many ways within the scopeof the following claims. The technical scope of the present inventioncan encompass any modifications obtainable by appropriately combiningtechnical means disclosed in different embodiments.

[1] Definition of Terms

(1-1) “Water Absorbent Resin”

What is meant by the wording “water absorbent resin” in the presentinvention is a water-swellable water-insoluble polymer gelling agent.Note that being “water swellable” is to have CRC (absorbency withoutpressure, defined in ERT 442.2-02) of 5 [g/g] or more, and being “waterinsoluble” is to have Ext (water soluble content, defined in ERT470.2-02) of 0 to 50 wt %.

The water absorbent resin can be designed as suitable for its purposes,and is not limited to a particular structure. It is, however, preferablethat the water absorbent resin be a hydrophilic crosslinked polymer inwhich an unsaturated monomer(s) having a carboxyl group iscrosslinkingly polymerized. Further, the water absorbent resin is notlimited to an embodiment in which the water absorbent resin is totally apolymer (100 wt %). As long as the above properties are ensured, thewater absorbent resin can be subjected to surface cross-linking or maycontain an additive or the like.

In the present invention, water absorbent resin obtained by grinding thehydrophilic crosslinked polymer into powder, which water absorbent resinhas not been subjected to a surface treatment or surface cross-linking,is, for convenience, referred to as “water absorbent resin particles”,whereas water absorbent resin obtained by grinding the hydrophiliccrosslinked polymer into powder, which water absorbent resin has beensubjected to a surface treatment or surface cross-linking, is referredto as “water absorbent resin powder”. Further, water absorbent resinhaving different shapes (such as shapes of sheet, fiber, film and gel)in steps, and a water absorbent resin composition including, forexample, an additive are referred to as “water absorbent resin”.

(1-2) “Polyacrylic Acid (Salt)”

In the present invention, the wording “polyacrylic acid (salt)” means apolymer, which may contain a graft component as appropriate, and whosemain component is acrylic acid and/or its salt (hereinafter, can bereferred to as acrylic acid (salt)) as its repeating unit. Morespecifically, what is meant by the “polyacrylic acid (salt)” is apolymer in which acrylic acid (salt) essentially accounts for 50 mol %to 100 mol % in the total monomer content (except an internalcrosslinking agent) to be polymerized, preferably a polymer in whichacrylic acid (salt) accounts for 70 mol % to 100 mol % in the totalmonomer content, more preferably a polymer in which acrylic acid (salt)accounts for 90 mol % to 100 mol % in the total monomer content, andespecially preferably a polymer in which acrylic acid (salt) accountsfor substantially 100 mol % in the total monomer content. Moreover, in acase where polyacrylic acid salt is used as a polymer, the polyacrylicacid salt essentially contains water-soluble salt, and a main componentof neutralization salt is preferably monovalent salt, more preferablyalkali metal salt or ammonium salt, further more preferably alkali metalsalt, and especially preferably sodium salt.

(1-3) “EDANA” and “ERT”

“EDANA” is abbreviation of European Disposables and NonwovensAssociations. “ERT” is abbreviation of EDANA Recommended Test Methods,which is a water-absorbable resin measuring method adopted as theEuropean standard (substantially global standard). In the presentinvention, unless otherwise specified, measurement is carried outaccording to the ERT master copy (Known Literature: 2002 revisedversion).

(a) “CRC” (ERT 441.2-02)

“CRC” stands for Centrifuge Retention Capacity, and means absorbencywithout pressure (hereinafter, may be refereed to as “absorbency”). Morespecifically, CRC is absorbency (unit; g/g) measured by allowing 0.200 gof water absorbent resin wrapped in unwoven cloth to freely swell with a0.9 wt % sodium chloride aqueous solution in a largely excess amount for30 minutes and then draining the water absorbent resin by using acentrifugal device. Note that CRC of a hydrogel crosslinked polymer(hereinafter referred to as “gel CRC”) was also measured under acondition where a sample and a free swelling time were changed to 0.4 gand 24 hours, respectively.

(b) “AAP” (ERT 442.2-02)

“AAP” stands for Absorption Against Pressure, and means absorbencymeasured under load. More specifically, AAP is absorbency (unit; g/g)measured by allowing 0.900 g of water absorbent resin to swell with a0.9 wt % sodium chloride aqueous solution for 1 hour under load of 2.06kPa (0.3 psi, 21 [g/cm²]). Note that AAP is referred to as AbsorptionUnder Pressure in ERT 442.2-02. AAP and AUP are substantially identicalwith each other. In the present invention and Example, AAP was measuredwith load of 4.83 kPa (0.7 psi, 49 [g/cm²]).

(c) “Ext” (ERT 470.2-02)

“Ext” stands for Extractables, and means water soluble content (watersoluble component amount). More specifically, Ext is water solublecontent (unit; wt %) measured by adding 1.000 g of water absorbent resininto 200 ml of 0.9 wt % sodium chloride aqueous solution, stirring for16 hours, and measuring, by pH titration, an amount of water solublecontent dissolved in a polymer. Note that water soluble content of ahydrogel crosslinked polymer (hereinafter referred to as “gel Ext”) wasalso measured under a condition where a sample and a stirring time werechanged to 5.0 g and 24 hours, respectively.

(d) “PSD” (ERT 420.2-02)

“PSD” stands for Particle Size Distribution, and means particle sizedistribution measured by classification by sieving. Here, a weightaverage particle diameter (D50) and a particle size distribution rangeare measured by the same method as one described in “(1) AverageParticle Diameter and Distribution of Particle Diameter” in page 7,lines 25 through 43 in the specification of EP Patent No. 0349240. Notethat a method for measuring PSD of a hydrogel crosslinked polymer willbe described later. A standard sieve (mesh size) to be used for particlesize measurement can be changed as appropriate in accordance with aparticle size of a target to be measured. For example, standard sieveshaving respective mesh sizes of, for example, 710 μm and 600 μm can beemployed. EP Patent No. 1594556 can be referred as appropriate formeasurement conditions, etc. which are not disclosed in EP Patent No.0349240.

(e) “Residual Monomers” (ERT410.2-02)

“Residual monomers” mean quantity of monomers left in water absorbentresin (hereinafter referred to as “residual monomers”). Specifically,residual monomers are quantity (unit; ppm) of monomers dissolved bystirring, at 500 rpm for 1 hour by use of a 35 mm stirrer chip, 200 mlof 0.9 wt % sodium chloride aqueous solution to which 1.0 g of waterabsorbent resin has been added. The quantity of residual monomers ismeasured by HPLC (high performance liquid chromatography). Note thatresidual monomers of a hydrogel crosslinked polymer were also measuredunder a condition where a sample and a stirring time were changed to 2 gand 3 hours, respectively. An obtained measurement value is convertedinto weight (unit; ppm) per resin solid content of the hydrogelcrosslinked polymer.

(f) “Moisture Content” (ERT430.2-02)

“Moisture content” means moisture content of water absorbent resin.Specifically, the moisture content (unit; wt %) is calculated fromdrying loss obtained by drying 1 g of water absorbent resin at 105° C.for three hours. Note that in the present invention, drying temperaturewas changed to 180° C., measurement was carried out 5 times for eachsample, and an average value calculated from the five measurements wasemployed. Moisture content of a hydrogel crosslinked polymer was alsomeasured under a condition where a sample, drying temperature, anddrying time were changed to 2 g, 180° C., and 16 hours, respectively.Note also that a value calculated by (100−moisture content (wt %)) is“resin solid content” in the present invention, and the resin solidcontent can be applied to the water absorbent resin and the hydrogelcrosslinked polymer.

(g) “Density” (ERT460.2-02)

“Density” means bulk specific gravity of water absorbent resin.Specifically, the density is weight (unit; [g/ml]) of water absorbentresin filling a 100 mL container into which 100 g of water absorbentresin which has been supplied into a device satisfying EDANA standardsis freely dropped.

(h) “Flow Rate” (ERT450.2-02)

“Flow rate” means a flow rate of water absorbent resin. Specifically,the flow rate is a period of time (unit; sec) required for discharging,from an opening in an undermost part of a device satisfying EDANAstandards, 100 g of water absorbent resin which has been supplied intothe device.

(1-4) “Permeability Potential”

“Permeability potential” of the present invention regards flowing of aliquid between particles of swollen gel under load or without load. The“permeability potential” is measured typically as SFC (Saline FlowConductivity) or GBP (Gel Bed Permeability).

“SFC” is permeability potential of water absorbent resin for 0.69 wt %sodium chloride aqueous solution under load of 2.07 kPa, and measuredaccording to the SFC test method described in U.S. Pat. No. 5,669,894.Moreover, “GBP” is permeability potential of water absorbent resin for0.69 wt % sodium chloride aqueous solution wherein the water absorbentresin is under load or allowed to freely swell. GBP is measuredaccording to the GBP test method described in PCT InternationalPublication No. 2005/016393.

(1-5) “FSR”

“FSR” of the present inventions is abbreviation of Free Swell Rate, andmeans a water absorbing rate (free swell rate). Specifically, FSR is arate (unit; [g/g/s]) at which 1 g of water absorbent resin absorbs 20 gof 0.9 wt % sodium chloride aqueous solution.

(1-6) “Gel Grinding”

“Gel Grinding” of the present invention means reducing, by shearingstress and compressive force, the size of a hydrogel crosslinked polymerobtained in a polymerization process (preferably aqueous polymerizationor unstirred aqueous polymerization (static aqueous polymerization),especially preferably belt polymerization) so as to increase a surfacearea of the hydrogel crosslinked polymer, so that the hydrogelcrosslinked polymer is easily dried. Specifically, the gel grindingmeans subjecting, to gel grinding, the hydrogel crosslinked polymerobtained in the polymerization step so as to have (i) a weight averageparticle diameter (D50) in a range of 300 μm to 3000 μm, and morepreferably in a range of 350 μm to 2000 μm, and (ii) logarithmicstandard deviation (σζ) of particle size distribution in a range of 0.2to 1.0.

Note that the hydrogel crosslinked polymer can have different shapesdepending on types of a polymerizer. In, for example, kneaderpolymerization, polymerization and gel grinding are continuously carriedout in an identical device, while in unstirred aqueous polymerization(static aqueous polymerization, particularly belt polymerization), gelgrinding is carried out after polymerization. The gel grinding can becarried out during or after polymerization provided that a particulatehydrogel crosslinked polymer to be dried have the following range ofweight average particle diameter (D50).

(1-7) “Weight Average Molecular Weight of Water Soluble Content”

“Weight average molecular weight of water soluble content” of thepresent invention is weight average molecular weight (unit; daltons,hereinafter simply referred to as [Da]) of a component (water solublecontent) of water absorbent resin, which component is dissolved in anaqueous medium. The weight average molecular weight is measured by GPC(gel permeation chromatography). That is, the weight average molecularweight is obtained by measuring, by GPC, a solution measured by themethod described in ‘(1-3) (c) “Ext”’. Note that weight averagemolecular weight of water soluble content of a hydrogel crosslinkedpolymer was also measured under a condition where (i) a sample having aparticle diameter of not more than 5 mm, further 1 mm through 3 mm and(ii) a stirring time were changed to 5.0 g and 24 hours, respectively.

(1-8) “Gel Grinding Energy” (GGE and GGE (2))

“Gel grinding energy” of the present invention means mechanical energyper unit weight (unit weight of a hydrogel crosslinked polymer), whichmechanical energy is necessary for a gel grinding device to grind thehydrogel crosslinked polymer. The gel grinding energy does not includeenergy for heating or cooling a jacket and energy of water and steam tobe supplied. Note that “Gel Grinding Energy” is abbreviated as “GGE”.GGE is calculated by the following expression (1) in a case where thegel grinding device is driven by a three-phase alternating current powersupply.

[Mathematical Expression 1]GEE [J/g]={√3×voltage×current×power factor×motor efficiency}/{weight ofhydrogel crosslinked polymer to be supplied into gel grinding device forone second}  Expression (1)

The “power factor” and the “motor efficiency” are respectivecharacteristic values for the gel grinding device in a range of 0 to 1,which values change depending on an operation condition etc. of the gelgrinding device. It is possible to know the characteristic values byinquiring them from a manufacturer etc. of the gel grinding device. In acase where the gel grinding device is driven by a single-phasealternating current power supply, GGE can be calculated by replacing“√3” with “1” in the above Expression (1). Note that voltage unit is[V], current unit is [A], and unit of weight of a hydrogel crosslinkedpolymer is [g/s].

It is preferable in the present invention that the gel grinding energybe calculated without a current value of the gel grinding device duringidling. This is because the mechanical energy to be applied to thehydrogel crosslinked polymer is important. Especially in a case where aplurality of gel grinding devices are employed, a total current value ofthe plurality of gel grinding devices during idling becomes great. It istherefore suitable to calculate GGE without the total current value. Thegel grinding energy is calculated by the following Expression (2). Notethat GGE calculated by Expression (2) is described as GGE (2) so as tobe separated from GGE calculated by Expression (1).

[Mathematical Expression 2]GGE (2) [J/g]={√3×voltage×(current during gel grinding−current duringidling)×power factor×motor efficiency}/{weight of hydrogel crosslinkedpolymer to be supplied into gel grinding device for one second}  Expression (2)

The “power factor” and the “motor efficiency” in GGE (2) are valuesduring gel grinding. Note that values of the power factor and the motorefficiency during idling are approximately defined as in Expression (2)because a current value during idling is small. The “weight of hydrogelcrosslinked polymer to be supplied into gel grinding device for onesecond [g/s]” in Expressions (1) and (2) is a conversion value of [t/hr]into [g/s] in a case where an amount of the hydrogel crosslinked polymerto be continuously supplied by a quantitative feeder is [t/hr].

(1-9) Others

In this specification, the expression “X to Y” for expressing a rangemeans “not less than X and not more than Y”. The weight unit “t (ton)”means “Metric ton”. Further, unless otherwise specified, “ppm” means“ppm by weight”. Moreover, “weight” and “mass”, “wt %” and “mass %”, and“parts by weight” and “parts by mass” are synonymous with each othercorrespondingly herein. Further, the wording “ . . . acid (salt)” means“ . . . acid and/or salt thereof”. The wording “(meth)acrylic” means“acrylic and/or methacrylic”. [2] Method For Producing Water AbsorbentPolyacrylic Acid (Salt) Resin Power

(2-1) Polymerization Step

The polymerization step is a step for polymerizing an aqueous solutionwhose main component is acrylic acid (salt) so as to obtain a hydrogelcrosslinked polymer (hereinafter may be referred to as “hydrogel”).

(Monomer)

Water absorbent resin powder to be produced in the present invention ismade from a material (monomers) whose main component is acrylic acid(salt). The water absorbent resin powder is normally polymerized in astate of an aqueous solution. Monomer concentration in a monomer aqueoussolution is preferably in a range of 10 wt % to 80 wt %, more preferablyin a range of 20 wt % to 80 wt %, further more preferably in a range of30 wt % to 70 wt %, and especially preferably in a range of 40 wt % to60 wt %.

It is preferable in terms of absorbency and residual monomers that thehydrogel obtained by the polymerization of the monomer aqueous solutionhave a polymer structure having acid groups, at least some of which areneutralized. Such partial neutralization salt is not limited to aspecific one but is, in terms of absorbency, preferably monovalent saltselected from a group consisting of alkali metal salt, ammonium salt,and amine salt, more preferably alkali metal salt, further morepreferably alkali metal salt selected from a group consisting of sodiumsalt, lithium salt, and potassium salt, and especially preferably sodiumsalt. Therefore, a basic substance to be used for such neutralization isnot limited to a specific one but is preferably a monovalent basicsubstance such as (i) a hydroxide of alkali metal including sodiumhydroxide, potassium hydroxide, and lithium hydroxide or (ii) carbonate(hydrogen carbonate) including sodium carbonate (hydrogen carbonate) andpotassium carbonate (hydrogen carbonate), and especially preferablysodium hydroxide.

The neutralization can be carried out in various ways and under variousconditions before, during, and after the polymerization. For example,hydrogel obtained by polymerizing unneutralized or low-neutralized (forexample, 0 through 30 mol % neutralized) acrylic acid can beneutralized, particularly neutralized while being ground. It is,however, preferable in terms of improvement in productivity, property,etc. that unpolymerized acrylic acid be neutralized. That is, it ispreferable that neutralized acrylic acid (partial neutralization salt ofacrylic acid) be used as a monomer.

A rate of the neutralization is not limited to a specific one but is, asa product of water absorbent resin, preferably in a range of 10 mol % to100 mol %, more preferably in a range of 30 mol % to 95 mol %, furthermore preferably in a range of 45 mol % to 90 mol %, and especiallypreferably in a range of 60 mol % to 80 mol %. Temperature of theneutralization is neither limited to a specific one but is preferably ina range of 10° C. to 100° C., and more preferably in a range of 30° C.to 90° C. As to other neutralization process conditions, the conditiondisclosed in EP Patent No. 574260 is preferably applied to the presentinvention. Note that it is preferable that the hydrogel having the rateof the neutralization in the above range be ground in the following gelgrinding step.

In order to improve properties of the water absorbent resin powder to beproduced in the present invention, it is possible to add an arbitrarycomponent such as (i) aqueous resin or water absorbent resin includingstarch, cellulose, polyvinyl alcohol (PVA), polyacrylic acid (salt), andpolyethyleneimine, (ii) a forming agent including carbonate, an azocompound, and air bubble, (iii) an interfacial active agent, or (iv) anadditive, to the monomer aqueous solution, the hydrogel, a driedpolymer, the water absorbent resin, etc. in a step of a productionprocess of the present invention. In a case where the aqueous resin orthe water absorbent resin is added, an amount of the aqueous resin orthe water absorbent resin to add is preferably in a range of 0 to 50 wt% with respect to monomer, more preferably in a range of 0 to 20 wt %,further more preferably in a range of 0 to 10 wt %, and especiallypreferably in a range of 0 to 3 wt %. In a case where the foaming agent,the interfacial active agent, or the additive is added, an amount of thefoaming agent, the interfacial active agent, or the additive to add ispreferably in a range of 0 to 5 wt %, and more preferably in a range of0 to 1 wt %. Note that a graft polymer or a water absorbent resincomposition can be obtained by addition of the aqueous resin or thewater absorbent resin. A polymer of starch and acrylic acid, a polymerof PVA and acrylic acid, and like polymer are also regarded as waterabsorbent polyacrylic acid (salt) resin in the present invention.

Further, a chelating agent, an α-hydroxycarboxylic compound, or aninorganic reducing agent can be used in order to improve (i) color tonestability of the water absorbent resin powder to be produced in thepresent invention (color tone stability of the water absorbent resinpowder which is stored for a long period of time under high temperatureand high humidity) and (ii) urine resistance (prevention of geldeterioration) of the water absorbent resin powder. Among these, thechelating agent is especially preferably used. An amount of thechelating agent, the a-hydroxycarboxylic compound, or the inorganicreducing to use is preferably in a range of 10 ppm through 5000 ppm withrespect to the water absorbent resin, more preferably in a range of 10ppm through 1000 ppm, further more preferably in a range of 50 ppmthrough 1000 ppm, and especially preferably in a range of 100 ppmthrough 1000 ppm. Note that the compounds disclosed in U.S. Pat. No.6,599,989 or PCT International Publication No. 2008/090961 are employedas the chelating agent of the present invention. Among the compounds, anaminocarboxylate metal chelating agent, and a polyvalent phosphatecompound are preferably employed.

In the present invention, in a case where acrylic acid (salt) isemployed as a main component, a hydrophilic or hydrophobic unsaturatedmonomer(s) (hereinafter referred to as “other monomer(s)”) other thanthe acrylic acid (salt) may be used in combination with the acrylic acid(salt). Such other monomer(s) is not limited to a specific one. Examplesof the other monomer(s) encompass methacrylic acid, (anhydrous) maleicacid, 2-(meth)acrylamide-2-methyl propanesulfonic acid,(meth)acryloxyalkanesulfonic acid, N-vinyl-2-pyrolidone,N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide,N,N-dimethyl(meth)acrylamide, 2-hydroxyethyl(meth)acylate,methoxypolyethyleneglycol(meth)acrylate,polyethyleneglycol(meth)acrylate, stearylacrylate, and salts thereof. Anamount of the other monomer(s) to use is determined as appropriate so asnot to impair absorbency of the water absorbent resin powder, and ispreferably, but not limited to, in a range of 0 to 50 mol % with respectto the total monomer weight, more preferably in a range of 0 to 30 mol%, and further more preferably in a range of 0 to 10 mol %.

(Internal Crosslinking Agent)

It is preferable in terms of the absorbency of the water absorbent resinpowder to be produced in the present invention that a crosslinking agent(hereinafter may be referred to as an “internal crosslinking agent”) beused. The internal crosslinking agent is not limited to a specific one.Examples of the internal crosslinking agent encompass a polymerizablecrosslinking agent which is polymerizable with acrylic acid, a reactivecrosslinking agent which is reactive with a carboxyl group, and acrosslinking agent which is polymerizable with acrylic acid and reactivewith a carboxyl group.

Examples of the polymerizable crosslinking agent encompass compoundseach having at least two polymerizable double bonds in a molecule, suchas N,N′-methylene bisacrylamide, (poly)ethylene glycol di(meth)acrylate, (polyoxyethylene)trimethylolpropane tri(meth)acrylate, andpoly(meth)allyloxy alkanes. Examples of the reactive crosslinking agentencompass (i) covalent bonding crosslinking agents such as polyglycidylether (e.g., ethyleneglycoldiglycidyl ether) and, polyvalent alcohol(e.g., propanediol, glycerine, and sorbitol), and (ii) an ionic bondingcrosslinking agent such as a polyvalent metal compound (e.g., aluminumsalt). Among these, in terms of the absorbency, the internalcrosslinking agent is more preferably the polymerizable crosslinkingagent which is polymerizable with acrylic acid, and especiallypreferably an acrylate, allyl or acrylamide polymerizable crosslinkingagent. One or more types of the internal crosslinking agents can beemployed. Note that in a case where the polymerizable crosslinking agentand the reactive crosslinking agent are used in combination, acombination ratio thereof is preferably 10:1 through 1:10.

An amount of the internal crosslinking agent to use is, in terms of theproperties, preferably in a range of 0.001 mol % to 5 mol % with respectto the total monomer weight excluding a crosslinking agent, morepreferably in a range of 0.002 mol % to 2 mol %, further more preferablyin a range of 0.04 mol % to 1 mol %, especially preferably in a range of0.06 mol % to 0.5 mol %, and most preferably in a range of 0.07 mol % to0.2 mol %. Moreover, in an especially preferable embodiment of thepresent invention, an amount of the polymerizable crosslinking agent touse is preferably in a range of 0.01 mol % to 1 mol %, more preferablyin a range of 0.04 mol % to 0.5 mol %, and further more preferably in arange of 0.07 mol % to 0.1 mol %.

(Polymerization Initiator)

The polymerization initiator for use in the present invention isselected as appropriate, considering how the polymerization is carriedout. Any polymerization initiator is applicable. For example, aphotolytic polymerization initiator, a pyrolysis polymerizationinitiator, a redox polymerization initiator, and the like can beexemplified.

Examples of the photolysis polymerization initiator encompass benzoinderivative, benzyl derivative, acetophenone derivative, benzophenonederivative, and azo compound. Moreover, examples of the pyrolysispolymerization initiator encompass (i) persulfates such as sodiumpersulfate, potassium persulfate, and ammonium persulfate, (ii)peroxides such as hydrogen peroxide, t-butyl peroxide, andmethyl-ethyl-ketone peroxide, and (iii) azo compounds such as2,2′-azobis(2-amidino propane) dihydrochloride, and2,2′-azobis[2-(2-imidazoline 2-yl) propane] dihydrochloride.Furthermore, examples of the redox polymerization initiator encompassmixtures having persulfate or peroxide together with a reduciblecompound, such as L-ascorbic acid or sodium hydrogensulfite incombination. Moreover, it is one preferable embodiment to use theabove-mentioned photolytic polymerization initiator and pyrolysispolymerization initiator in combination.

An amount of the polymerization initiator to use is preferably in arange of 0.0001 mol % to 1 mol %, and more preferably in a range of0.0005 mol % to 0.5 mol %, with respect to the total monomer weight. Ina case where the amount of the polymerization initiator exceeds 1 mol %,the polymerization initiator would adversely affect a color tone of thewater absorbent resin. Moreover, it is not preferable that the amount ofthe polymerization initiator is less than 0.0001 mol %, because, if so,this would result in increase in residual monomers.

(Polymerization Method)

Particulate hydrogel can be polymerized by spraying dropletpolymerization or reverse-phase suspension polymerization in the methodfor producing the water absorbent resin powder of the present invention.Meanwhile, in view of the permeability potential (SFC) and the waterabsorbing rate (FSR) of the water absorbent resin powder, and in orderto easily control the polymerization, aqueous polymerization is carriedout. The aqueous polymerization can be tank-type (silo-type) unstirringpolymerization but preferably kneader polymerization or beltpolymerization, more preferably continuous aqueous polymerization,further more preferably high-concentration continuous aqueouspolymerization, and especially preferably high-concentrationhigh-temperature starting continuous aqueous polymerization. Note herethat what is meant by stirring polymerization is polymerizing carriedout under stirring of the hydrogel, especially under stirring andgrinding of the hydrogel (wherein the hydrogel is particularly hydrogelhaving a polymerization ratio of not less than 10 mol %, furtherparticularly hydrogel having a polymerization ratio of not less than 50mol %). The stirring of the monomer aqueous solution (having apolymerization ratio of less than 0 to 10 mol %) may be carried out asappropriate before and/or after the unstirring polymerization.

Examples of the continuous aqueous polymerization encompass continuouskneader polymerization (disclosed in U.S. Pat. Nos. 6,987,171 and6,710,141, etc.), and continuous belt polymerization (disclosed in U.S.Pat. Nos. 4,893,999 and 6,241,928, US Patent Application Publication No.2005/215734, etc.). These aqueous polymerizations can produce the waterabsorbent resin powder with high productivity.

In the high-concentration continuous aqueous polymerization, monomerconcentration (solid content) is preferably not less than 35 wt %, morepreferably not less than 40 wt %, and further more preferably not lessthan 45 wt % (but not more than saturated concentration). Inhigh-temperature starting continuous aqueous polymerization,polymerization starting temperature is preferably not less than 30° C.,more preferably not less than 35° C., further more preferably not lessthan 40° C., and especially preferably not less than 50° C. (but notmore than boiling temperature). The high-concentration high-temperaturestarting continuous aqueous polymerization is a combination of thehigh-concentration continuous aqueous polymerization and thehigh-temperature starting continuous aqueous polymerization.

The high-concentration high-temperature starting continuous aqueouspolymerization (disclosed in U.S. Pat. Nos. 6,906,159 and 7,091,253,etc.) is preferable because it can produce the water absorbent resinpowder with a high degree of whiteness, and can be easily applied toindustrial-scale production.

Therefore, the polymerization method in the production method of thepresent invention is suitably applicable to a large-scale productiondevice having a great production volume per a production line. Note herethat the production volume is preferably not less than 0.5 [t/hr], morepreferably not less than 1 [t/hr], further more preferably 5 [t/hr], andespecially preferably 10 [t/hr].

The polymerization may be carried out under air atmosphere. It is,however, preferable in terms of coloring prevention that thepolymerization be carried out under inert gas atmosphere such as watervapor, nitrogen, or argon (with, for example, an oxygen concentration ofnot more than 1 volume %). It is further preferable that thepolymerization be carried out after oxygen dissolved in a monomer(s) orin a solution containing a monomer(s) is substituted for (deaeratedwith) inert gas (by, for example, less than 1 [mg/L] of oxygen). Bycarrying out such deaeration, it is possible to provide the waterabsorbent resin powder with a further excellent property and a highdegree of whiteness, in which water absorbent resin powder themonomer(s) has an excellent stability, and gelling is not caused beforepolymerization.

(2-2) Gel Grinding Step

The gel grinding step is a step for grinding the hydrogel crosslinkedpolymer during or after polymerization to obtain a particulate hydrogelcrosslinked polymer (hereinafter can be referred to as “particulatehydrogel”). Note that the gel grinding step is called “gel grinding” soas to be separated from “grinding” in “(2-4) Grinding Step andClassifying Step”.

(Property of Hydrogel Before Gel Grinding)

According to the method for producing the water absorbent resin powderof the present invention (first producing method), the gel grindingenergy (GGE) is controlled in a specific range. In the first producingmethod, it is preferable to subject, to gel grinding, a hydrogelcrosslinked polymer (polyacrylic acid (salt) crosslinked polymer), atleast one of which gel temperature, resin solid content, gel CRC, gelExt, and weight average molecular weight of water soluble content iscontrolled in the following range.

That is, the method for producing the water absorbent resin powder ofthe present invention (first producing method) is, for example, a methodfor producing water absorbent polyacrylic acid (salt) resin powder,including the steps of: (i) polymerizing an acrylic acid (salt) monomeraqueous solution; (ii) during or after the step of (i), performing gelgrinding of a hydrogel crosslinked polymer obtained by thepolymerization, wherein the hydrogel crosslinked polymer has resin solidcontent of 10 wt % to 80 wt %, and the gel grinding is carried out withgel grinding energy (GGE) of 18 [J/g] to 60 [J/g]; (iii) drying aparticulate hydrogel crosslinked polymer obtained by the gel grinding,wherein the drying is performed at 150° C. to 250° C.; and (iv) carryingout a surface treatment to the particulate hydrogel crosslinked polymerthus dried.

According to the method for producing the water absorbent resin powderof the present invention (second producing method), the gel grindingenergy (2) (GGE (2)) is controlled in a specific range. In the secondproducing method, it is preferable to subject, to gel grinding, ahydrogel crosslinked polymer (polyacrylic acid (salt) crosslinkedpolymer), at least one of which gel temperature, resin solid content,gel CRC, gel Ext, and weight average molecular weight of water solublecontent is controlled in the following range.

The method for producing the water absorbent resin powder of the presentinvention (second producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization, wherein the hydrogelcrosslinked polymer has resin solid content of 10 wt % to 80 wt %, andthe gel grinding is carried out with gel grinding energy (2) (GGE (2))of 9 [J/g] to 40 [J/g]; (iii) drying a particulate hydrogel crosslinkedpolymer obtained by the gel grinding, wherein the drying is performed at150° C. to 250° C.; and (iv) carrying out a surface treatment to theparticulate hydrogel crosslinked polymer thus dried.

Note that conventional gel grinding techniques are mainly techniques ofapplying shearing stress as less as possible (see U.S. Pat. Nos.7,694,900, 6,565,768 and 6,140,395, etc.). In contrast, a gel grindingtechnique of the present invention is characteristic in applyingshearing stress greater than conventional shearing stress so as toincrease the weight average molecular weight of the water solublecontent.

(a) Gel Temperature

For the sake of particle diameter control and properties, temperature(gel temperature) of hydrogel before gel grinding is preferably in arange of 40° C. to 120° C., more preferably in a range of 60° C. to 120°C., further more preferably in a range of 60° C. to 110° C., andespecially preferably in a range of 65° C. to 110° C. A gel temperaturelower than 40° C. results in a greater hardness of the resultanthydrogel, thereby making it difficult to control a particle shape andparticle size distribution in grinding. Moreover, a gel temperaturehigher than 120° C. results in a greater softness of the resultanthydrogel on the contrary, thereby making it difficult to control theparticle shape and the particle size distribution. The gel temperaturecan be controlled appropriately by the polymerization temperature,post-polymerization heating, heat-retaining or cooling, etc.

(b) Resin Solid Content

Resin solid content of hydrogel before gel grinding is, in terms ofproperties, in a range of 10 wt % to 80 wt %, preferably in a range of30 wt % to 80 wt %, more preferably in a range of 40 wt % to 80 wt %,further more preferably in a range of 45 wt % to 60 wt %, and especiallypreferably in a range of 50 wt % to 60 wt %. Resin solid content lessthan 10 wt % is not preferable because it results in a greater softnessof the resultant hydrogel, thereby making it difficult to control theparticle shape and the particle size distribution. Moreover, resin solidcontent greater than 80 wt % is neither preferable because it results ina greater hardness of the resultant hydrogel on the contrary, therebymaking it difficult to control the particle shape and the particle sizedistribution. The resin solid content of the hydrogel can beappropriately controlled by polymerization concentration, moisturevaporization during polymerization, addition of water absorbent resinfine powder (fine powder recycling step) in a polymerization step, or ifnecessary, moisturization or partial drying after polymerization.

Note that the resin solid content of the hydrogel before the gelgrinding is calculated from the drying loss described in (f) of (1-3)after the hydrogel is cut or fragmented by use of scissors, a cutter, orthe like so as to have a side of not more than 5 mm, preferably a sideof 1 mm through 3 mm. Gel grinding energy during the cutting by use ofthe scissors, the cutter, or the like is substantially zero.

(c) Gel CRC

CRC (gel CRC) of hydrogel before gel grinding is preferably in a rangeof 10 [g/g] to 35 [g/g], more preferably in a range of 10 [g/g] to 32[g/g], and further more preferably in a range of 10 [g/g] to 30 [g/g],and especially preferably in a range of 15 [g/g] to 30 [g/g]. Gel CRCless than 10 [g/g] or more than 35 [g/g] is not preferable because itmakes it difficult to control the particle shape and the particle sizedistribution during the gel grinding. The gel CRC of the hydrogel beforethe gel grinding can be appropriately controlled by an amount ofcrosslinking agent to add during polymerization, polymerizationconcentration, or the like. Note that it is conventionally well-knownthat it is preferable that water absorbent resin have a high CRC. Itwas, however, found in the present invention that the gel CRC more than35 [g/g] makes it difficult to control the particle shape and theparticle size distribution.

Note that the gel CRC of the hydrogel before the gel grinding iscalculated by a measurement method that will be described in (a) of[Examples] after the hydrogel is cut or fragmented by use of scissors, acutter, or the like so as to have a side of not more than 5 mm,preferably a side of 1 mm through 3 mm.

(d) Gel Ext

Water soluble content (gel Ext) of hydrogel before gel grinding ispreferably in a range of 0.1 wt % to 10 wt %, more preferably in a rangeof 0.5 wt % to 8 wt %, and further more preferably in a range of 1 wt %to 5 wt %. Gel Ext more than wt % results in excessive increase inweight average molecular weight of water soluble content due to shearingstress during the gel grinding, thereby failing to attain a desiredpermeability potential. The gel Ext of the hydrogel before the gelgrinding is preferably small. However, a lower limit of the gel Extshould be in the above range in terms of balance with the above (c) GelCRC, a manufacturing cost necessary for reduction in the gel Ext,decline in productivity, etc.

Note that the gel Ext of the hydrogel before the gel grinding iscalculated by a measurement method that will be described in (b) of[Examples] after the hydrogel is cut or fragmented by use of scissors, acutter, or the like so as to have a side of not more than 5 mm,preferably a side of 1 mm through 3 mm.

(e) Weight Average Molecular Weight of Water Soluble Content

Weight average molecular weight of water soluble content of hydrogelbefore gel grinding is preferably in a range of 50,000 [Da] to 450,000[Da], more preferably in a range of 100,000 [Da] to 430,000 [Da], andfurther more preferably in a range of 150,000 [Da] to 400,000 [Da].

Weight average molecular weight of water soluble content of less than50,000 [Da] results in reduction in particle diameter of particulatehydrogel obtained after the gel grinding, thereby making it impossibleto produce the water absorbent resin powder having a desired property.Moreover, hydrogel having weight average molecular weight of watersoluble content of more than 450,000 [Da] has less crosslinking points,and is damaged by shearing stress more than necessary. This possiblydeteriorates properties such as increase in the water soluble contentafter the gel grinding. The weight average molecular weight of the watersoluble content of the hydrogel before the gel grinding can beappropriately controlled by, for example, an amount of crosslinkingagent to add during polymerization, polymerization concentration, or ifnecessary, a chain transfer agent.

Note that the weight average molecular weight of the water solublecontent of the hydrogel before the gel grinding is calculated by ameasurement method that will be described in (c) of [Examples] after thehydrogel is cut or fragmented by use of scissors, a cutter, or the likeso as to have a side of not more than 5 mm, preferably a side of 1 mmthrough 3 mm.

(Gel Grinding Device)

Any kinds of gel grinding devices are applicable to the gel grindingstep, for example, a gel grinder having a plurality of rotationalstirring blades such as a batch-type or continuous double-armed kneader,a single- or twin-screw extruder, a meat chopper, particularly a screwextruder, etc. can be adopted.

Among these devices, the screw extruder having a porous die at an end ofa casing is preferable. For example, a screw extruder disclosed inJapanese Patent Application Publication, Tokukai, No. 2000-63527 A canbe adopted. The following describes a screw extruder with reference toFIG. 1.

The screw extruder illustrated in FIG. 1 includes a casing 11, a base12, a screw 13, a feed opening 14, a hopper 15, an extrusion opening 16,a porous die 17, a rotational blade 18, a ring 19, a backflow preventingmember 20, a motor 21, and a linear projection 22. The casing 11 has acylindrical shape. The screw 13 is provided in the casing 11. Theextrusion opening 16 where ground hydrogel is extruded out is formed atan edge of the casing 11. The porous die 17 is provided immediatelybefore the extrusion opening 16 in an extrusion direction in the casing11. The motor 21 for rotating the screw 13, a driving system, etc. areprovided at the other edge of the casing 11. The base 12 is under thecasing 11 so as to stably install the screw extruder. The feed opening14 for feeding hydrogel is provided above the casing 11. The hopper 15is provided so as to easily feed hydrogel. A shape and a size of thecasing 11 are not particularly limited provided that the casing 11 has acylindrical inner surface that fits a shape of the screw 13. The numberof revolutions of the screw 13 is not particularly limited because itvaries depending on the shape of the screw extruder. It is, however,preferable to change the number of revolutions of the screw 13 asdescribed later. The screw extruder illustrated in FIG. 1 can thusinclude the backflow preventing member 20 in the vicinity of theextrusion opening 16, and the linear projection 22 provided with thescrew 13. Configurations, materials, and sizes of the members, materialsfor the backflow preventing member 20 and various rotational blades ofthe screw 13, and all other configurations associated with the screwextruder, can be selected on the basis of a method disclosed in JapanesePatent Application Publication, Tokukai No. 2000-63527 A.

A configuration of the backflow preventing member 20 is not particularlylimited provided that the backflow preventing member 20 is configured toprevent backflow of hydrogel in the vicinity of the extrusion opening16. Examples of the backflow preventing member 20 encompass (i) a spiralor concentric strip projection provided on an inner wall of the casing11 and (ii) a strip, granular, spherical, or angular projection providedin parallel to the screw 13. Pressure is increased in the vicinity ofthe extrusion opening 16 as gel grinding proceeds. This causes hydrogelto flow back in a direction to the feed opening 14. However, byproviding the backflow preventing member 20, it is possible to subjectthe hydrogel to the gel grinding while preventing the backflow of thehydrogel.

(Porous Die)

A thickness, a porous diameter, and a hole area rate of the porous dieprovided at an outlet of a cylindrical body (casing) of the gel grindingdevice are not particularly limited, and can be appropriately selectedin accordance with, for example, how much the gel grinding deviceprocesses hydrogel per unit time, properties of the hydrogel. However,the thickness of the porous die is preferably in a range of 3.5 mm to 40mm, and more preferably in a range of 6 mm to 20 mm. The porous diameterof the porous die is preferably in a range of 3.2 mm to 24 mm, and morepreferably in a range of 7.5 mm to 24 mm. The hole area rate of theporous die is preferably in a range of 20% to 80%, and more preferablyin a range of 30% to 55%. Note that in a case where a plurality ofporous dies different in porous diameter (mm) are employed, a simpleaverage value of porous diameters of the respective plurality of porousdies is regarded as the porous diameter of the porous die of the gelgrinding device. Holes of the porous die each preferably have a circularshape. However, the hole can have a shape other than the circular shape(for example, a rectangular, elliptic, or slit shape). In a case wherethe hole has the shape other than the circular shape, a porous diameter(mm) of the hole having the shape other than the circular shape iscalculated by converting a hole area of the hole having the shape otherthan the circular shape into a circle.

It is not preferable that the porous die have at least one of (i) athickness of less than 3.5 mm, (ii) a porous diameter of more than 24 mmand (iii) a hole area rate of more than 80%. This is because such aporous die cannot sufficiently apply shearing stress and compressiveforce to the hydrogel. It is neither preferable that the porous die haveat least one of (i) a thickness of more than 40 mm, (ii) a porousdiameter of less than 3.2 mm and (iii) a hole area rate of less than20%. This is because such a porous die, in contrast, applies excessiveshearing stress and compressive force to the hydrogel, whereby theproperties of the hydrogel may be deteriorated.

(Gel Grinding Energy (GGE)/Gel Grinding Energy (2) (GGE 2))

According to the method for producing the water absorbent resin powderof the present invention, the gel grinding is carried out with the gelgrinding energy (GGE) controlled in a specific range. Note here that theGGE is controlled by, for example, the above-described method, and thegel grinding is preferably carried out with respect to the hydrogel(polyacrylic acid (salt) crosslinked polymer) at least one of whichproperties before the gel grinding, particularly the gel temperature,the gel CRC, the gel Ext, and the weight average molecular weight of thewater soluble content in addition to the resin solid content of 10 wt %to 80 wt % (further, the above (b)) is controlled in the above range. Itis possible to obtain particulate hydrogel having the following particlediameter by such gel grinding, that is, by a fourth producing method ofthe present invention, in addition to (i) the first producing method(the gel grinding with the GGE), (ii) the second producing method (thegel grinding with the GGE (2)), and (iii) a third producing method ofthe present invention (the gel grinding with the increase in the weightaverage molecular weight of the water soluble content), which fourthproducing method is carried out concurrently or non-concurrently withthe first through third producing methods.

In the present invention, an upper limit of the gel grinding energy(GGE) for grinding the hydrogel is preferably 60 [J/g] or less, morepreferably 50 [J/g] or less, and further more preferably 40 [J/g] orless. A lower limit of the GGE is preferably 18 [J/g] or more, morepreferably 20 [J/g] or more, and further more preferably 25 [J/g] ormore. Therefore, in the present invention, the gel grinding energy (GGE)for grinding the hydrogel is, for example, in a range of 18 [J/g] to 60[J/g], preferably in a range of 20 [J/g] to 50 [J/g], and morepreferably in a range of 25 [J/g] to 40 [J/g]. Controlling the GGE inthe above range makes it possible to grind the hydrogel while applyingappropriate shearing stress and compressive force to the hydrogel. Notethat the gel grinding energy (GGE) which includes energy during idlingof the gel grinding device is defined.

According to the second producing method of the present invention, thegel grinding energy (2) that excludes the energy during idling of thegel grinding device can be defined. That is, in the present invention,an upper limit of the gel grinding energy (2) (GGE (2)) for grinding thehydrogel is preferably 40 [J/g] or less, more preferably 32 [J/g] orless, and further more preferably 25 [J/g] or less. A lower limit of theGGE (2) is preferably 9 [J/g] or more, more preferably 12 [J/g] or more,and further more preferably 15 [J/g] or more. Therefore, in the presentinvention, the gel grinding energy (2) (GGE (2)) for grinding thehydrogel is, for example, in a range of 9 [J/g] to 40 [J/g], preferablyin a range of 12 [J/g] to 32 [J/g], and more preferably in a range of 15[J/g] to 25 [J/g]. Controlling the GGE (2) in the above range makes itpossible to grind the hydrogel while applying appropriate shearingstress and compressive force to the hydrogel.

By drying, under a specific condition, the particulate hydrogel obtainedin the gel grinding step, it is possible to improve the shape of thewater absorbent resin, and attain both high permeability and a waterabsorbing rate. Note that in a case where the hydrogel is ground by aplurality of devices such as by a screw extruder after kneaderpolymerization or by a plurality of screw extruders, the total of energyconsumed by the plurality of devices is the gel grinding energy (GGE) orthe gel grinding energy (2) (GGE (2)) of the present invention.

(Gel Grinding Region)

In the present invention, the gel grinding is carried out during orafter polymerization but more preferably carried out with respect to thehydrogel after polymerization. Note that in a case where the gelgrinding is carried out during polymerization such as kneaderpolymerization, a monomer aqueous solution which “is sufficientlygelling” is to be ground in the gel grinding step.

For example, in a case where kneader polymerization is carried out, amonomer aqueous solution changes to hydrogel as polymerization timeelapses. That is, the following regions sequentially appear: (i) aregion where the monomer aqueous solution is stirred at the start ofpolymerization, (ii) a region where low-polymerized hydrogel having aconstant viscosity is stirred during the polymerization, (iii) a regionwhere a part of the hydrogel is started to be ground as thepolymerization proceeds, and (iv) a gel grinding region in the secondhalf of or at the last stage of the polymerization. Therefore, in orderto clearly separate “stirring of the monomer aqueous solution” at thestart of the polymerization from the “gel grinding” at the last stage ofthe polymerization, the monomer aqueous solution that is “sufficientlygelling” is determined to be ground in the gel grinding step.

What is meant by “sufficiently gelling” is a state in which the hydrogelcan be ground by shearing stress after a maximum polymerizationtemperature (polymerization peak temperature). Alternatively,“sufficiently gelling” means a state in which the hydrogel can be groundby shearing stress after a polymerization ratio of monomers in themonomer aqueous solution (the polymerization ratio is also known asconversion ratio, and calculated from (i) polymer quantity calculated bypH titration of the hydrogel and (ii) residual monomer quantity) becomespreferably not less than 90 mol %, more preferably not less than 93 mol%, further more preferably not less than 95 mol %, and especiallypreferably not less than 97 mol %. That is, the hydrogel, in which thepolymerization ratio of the monomers is in the above range, is ground inthe gel grinding step of the present invention. Note that in a case ofpolymerization reaction that does not have the polymerization peaktemperature (such as a case where polymerization always proceeds at aconstant temperature, or a case where polymerization temperature keepsrising), whether or not the monomer aqueous solution is “sufficientlygelling” is determined on the basis of the polymerization ratio of themonomers.

Therefore, in a case where batch-type kneader polymerization is carriedout, the GGE during the batch-type kneader polymerization is measuredafter the polymerization peak temperature or the conversion ratio. In acase where continuous kneader polymerization is carried out, the GGE iscalculated by multiplying, by total GGE in the whole polymerizationstep, a ratio of polymerization time after the polymerization peaktemperature or the conversion ratio to a total polymerization time (seeExpression (3)).

[Mathematical Expression 3]GGE [J/g]=(total GGE)×(polymerization time after polymerization peaktemperature or conversion ratio)/(total polymerization time)  Expression(3)

Note that even in a case where a batch-type or continuous kneaderpolymerization device is employed, gel grinding can be separatelycarried out after the kneader polymerization. In the case, the total of(i) energy consumed by a device for performing the gel grinding and (ii)the GGE or the GGE (2) during the kneader polymerization is regarded asthe GGE or the GGE (2) of the present invention.

In a case where belt polymerization is carried out in the polymerizationstep, hydrogel during or after the belt polymerization, preferably afterthe belt polymerization, can be cut or ground to have a size of severaltens of centimeters before the gel grinding. Such cut or ground hydrogelis easily supplied into the gel grinding device. This makes it possibleto smoothly carry out the gel grinding step. Note that the hydrogel ispreferably cut or ground without being kneaded, and therefore cut orground by, for example, a guillotine cutter. A size and a shape of thecut or ground hydrogel are not particularly limited provided that it canbe supplied into the gel grinding device. Further, in a case whereweight of a piece of the ground hydrogel is one tenth or less of “weightof a hydrogel crosslinked polymer to be supplied into a gel grindingdevice per minute”, energy during grinding of the hydrogel is includedin the GGE during the gel grinding.

(Operation Condition of Gel Grinding Device)

In a case where the gel grinding device used in the gel grinding step ofthe present invention is a screw extruder, the number of revolutions ofa screw axis of the screw extruder cannot be simply defined. This isbecause a rate of a periphery of a rotational blade varies depending onan internal diameter of a cylindrical body (casing) of the screwextruder. However, the number of revolutions is preferably in a range of90 rpm to 500 rpm, more preferably in a range of 100 rpm to 400 rpm, andfurther more preferably in a range of 120 rpm to 200 rpm. The number ofrevolutions of less than 90 rpm is not preferable because the rotationalblade having such number of revolutions fails to apply shearing stressand compressive force required for the gel grinding. The number ofrevolutions of more than 500 rpm is neither preferable because therotational blade having such number of revolutions, in contrast, appliestoo much shearing stress and compressive force to the hydrogel. Thisdeteriorates the properties of the hydrogel, or increases load on thegel grinding device thereby damaging the gel grinding device. The rateof the periphery of the rotational blade is preferably in a range of 0.5[m/s] to 5 [m/s], and more preferably in a range of 0.5 [m/s] to 4[m/s]. Further, the gel grinding device of the present inventions isheated or kept to have a temperature preferably in a range of 40° C. to120° C., and more preferably in a range of 60° C. to 100° C., so as toprevent the hydrogel from adhering thereto.

(Use of Water)

In the gel grinding step of the present invention, the hydrogel to whichwater has been added can be ground. Note that in the present invention,the “water” can be in the form of a solid, liquid or gas.

How and when water is added are not particularly limited provided thatthe water is supplied to the gel grinding device in which the hydrogelis present. Alternatively, hydrogel to which water has been alreadyadded can be supplied to the gel grinding device. Further, it ispossible to add not only the water but also another additive (such as aninterfacial active agent, a base for neutralization, a crosslinkingagent, or inorganic salt) or a solvent other than the water. Note thatin a case where an additive or the solvent other than water is furtheradded, water content is preferably in a range of 90 wt % to 100 wt %,more preferably in a range of 99 wt % to 100 wt %, and further morepreferably approximately 100 wt %.

In the present invention, the water can be in the form of a solid,liquid or gas. It is, however, preferable in terms of easy handling thatthe water be in the form of liquid and/or gas. An amount of the water toadd is preferably in a range of 0 to 4 parts by weight with respect to100 parts by weight of hydrogel, and more preferably in a range of 0 to2 parts by weight. If the amount of water to add exceeded 4 parts byweight, then it would possibly cause a defect such as failure of dryingresulting in undried hydrogel.

In a case where the water is in the form of liquid, temperature of thewater to add is preferably in a range of 10° C. to 100° C., and morepreferably in a range of 40° C. to 100° C. In a case where the water isin the form of gas, temperature of the water to add is preferably in arange of 100° C. to 220° C., more preferably in a range of 100° C. to160° C., and further more preferably in a range of 100° C. to 130° C.Note that how to prepare the water in the form of gas is notparticularly limited. The water in the form of gas can be, for example,water vapor generated by heating of a boiler or gaseous water generatedfrom a surface of water by vibrating the water by ultrasonic waves.Further, the water in the form of gas is preferably water vapor having apressure higher than atmospheric pressure, and more preferably watervapor generated by a boiler.

(Use of Additive)

It is thus preferable to grind the hydrogel to which water has beenadded. Further, not only water but also an additive, a neutralizingagent, or the like can be added to and mixed with the hydrogel beforethe gel grinding. The resultant water absorbent resin can be modified.Specifically, an aqueous solution containing the basic substancedescribed in the above (2-1) (for example, a 10 wt % to 50 wt % sodiumhydroxide aqueous solution) can be added so as to neutralize(particularly in the above-described range of the neutralization rate)the hydrogel during the gel grinding. Alternatively, water absorbentresin fine powder (0.1 wt % to 30 wt % of water absorbent resin finepowder with respect to resin solid content) can be added so that finepowder recycling is carried out. Further, 0.001 wt % to 3 wt % of apolymerization initiator, a reducing agent or a chelating agent (withrespect to resin solid content) can be added to and mixed with thehydrogel during the gel grinding so as to reduce residual monomers,improve coloring, and attain endurance.

(Property of Particulate Hydrogel After Gel Grinding)

According to the method for producing the water absorbent resin powderof the present invention (third producing method), the weight averagemolecular weight of the water soluble content of the hydrogel isincreased to 10,000 [Da] to 500,000 [Da] by means of a production methodin which the gel grinding energy (GGE) is in a range of 18 [J/g] to 60[J/g] during the gel grinding of the hydrogel.

That is, in order to attain the object of the present invention, themethod for producing the water absorbent resin powder of the presentinvention (third producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization, wherein the hydrogelcrosslinked polymer has resin solid content of 10 wt % to 80 wt %, sothat weight average molecular weight of water soluble content of thehydrogel crosslinked polymer is increased by 10,000 [Da] to 500,000[Da]; (iii) drying a particulate hydrogel crosslinked polymer obtainedby the gel grinding, wherein the drying is performed at 150° C. to 250°C.; and (iv) carrying out a surface treatment to the particulatehydrogel crosslinked polymer thus dried.

The method for producing the water absorbent resin powder of the presentinvention (fourth producing method) is carried out such that theparticulate hydrogel crosslinked polymer thus obtained by the gelgrinding has a weight average particle diameter (D50) of 350 μm to 2000μm, logarithmic standard deviation (σζ) of particle size distribution of0.2 to 1.0, and resin solid content of 10 wt % to 80 wt %.

That is, in order to attain the object of the present invention, themethod for producing water absorbent resin powder of the presentinvention (fourth producing method) is a method for producing waterabsorbent polyacrylic acid (salt) resin powder, including the steps of:(i) polymerizing an acrylic acid (salt) monomer aqueous solution; (ii)during or after the step of (i), performing gel grinding of a hydrogelcrosslinked polymer obtained by the polymerization so as to obtain aparticulate hydrogel crosslinked polymer having a weight averageparticle diameter (D50) of 350 μm to 2000 μm and logarithmic standarddeviation (σζ) of particle size distribution of 0.2 to 1.0; (iii) dryingthe particulate hydrogel crosslinked polymer by hot air of 150° C. to250° C. at a velocity of 0.8 [m/s] to 2.5 [m/s] in a direction vertical(up-and-down direction) to the particulate hydrogel crosslinked polymerby use of a through-flow belt drier, the particulate hydrogelcrosslinked polymer to be supplied into the through-flow belt drierhaving resin solid content of 10 wt % to 80 wt %; and (iv) carrying outa surface treatment to the particulate hydrogel crosslinked polymer thusdried.

(a) Particle Diameter

The hydrogel crosslinked polymer (hydrogel) obtained in thepolymerization step is ground into particles by the gel grinding device(such as a kneader, a meat chopper, or a screw extruder) for performingthe gel grinding of the present invention. Note that the diameter of theparticles can be controlled by classification, blending, or the like. Itis, however, preferable that the diameter be controlled by the gelgrinding of the present invention.

The weight average particle diameter (D50) (defined by classification bysieving) of the particulate hydrogel after the gel grinding is in therange of 350 μm to 2000 μm, more preferably in a range of 400 μm to 1500μm, and further more preferably in a range of 500 μm to 1000 μm. Thelogarithmic standard deviation (σζ) of the particle size distribution isin the range of 0.2 to 1.0, more preferably in a range of 0.2 to 0.8,and further more preferably in a range of 0.2 to 0.7.

In a case where the weight average particle diameter is more than 2000μm, the hydrogel may have been ground by uneven or insufficient shearingstress and compressive force. Further, in the case, an inner part of thehydrogel is different from a surface of the hydrogel in degree ofdrying. This causes particles with an inhomogeneous property to begenerated by grinding after drying the hydrogel, thereby deterioratingthe whole property of the hydrogel. Moreover, in a case where the weightaverage particle diameter is less than 350 μm, the hydrogel has had anincreased surface area, and is therefore remarkably easily dried. Thismakes it insufficient to reduce residual monomers in the drying step,thereby increasing the residual monomers (later described in (3-5)).Further, this not only causes a large amount of fine powder to begenerated during the gel grinding after the drying, and makes itdifficult to control the particle diameter described in (2-4) (laterdescribed) but also deteriorates the properties of the hydrogel such aspermeability potential (SFC). Further, it is difficult to grind thehydrogel to a weight average particle diameter of less than 350 μm justby means of a normal gel grinding operation, and it is necessary toseparately carry out a special operation such as (i) classification ofgel after the gel grinding (see Japanese Patent Application Publication,Tokukaihei No. 6-107800 A etc.) or (ii) particle diameter control duringthe polymerization before the gel grinding (see, for example, EP PatentNo. 0349240 disclosing a method for producing gel particles having asharp particle size distribution during reverse-phase suspensionpolymerization). The special operation thus carried out in addition tothe gel grinding newly causes problems that (i) a large amount ofinterfacial active agent or organic solvent is required for thepolymerization or the classification and (ii) productivity is decreased(rise in cost) or the properties of the hydrogel are deteriorated(increase in residual monomers or fine power), and like problems. It istherefore not only difficult but also unpreferable that the particulatehydrogel has the weight average particle diameter of less than 350 μm.

It is preferable in terms of uniform drying that the logarithmicstandard deviation (σζ) be as small as possible. However, forlogarithmic standard deviation (σζ) of less than 0.2, it is necessary tocarry out the special operation such as the classification of the gelafter the gel grinding or the particle diameter control during thepolymerization before the gel grinding, as with the weight averageparticle diameter. It is therefore neither preferable nor attainable inconsideration of productivity and cost that the particulate hydrogel hasthe logarithmic standard deviation (σζ) of less than 0.2. The particlediameter can be controlled by the gel grinding of the present invention.The hydrogel is ground by particularly the screw extruder so as to havethe particle diameter.

(b) Gel CRC After Gel Grinding

In the present invention, gel CRC of the particulate hydrogel after thegel grinding is preferably in a range of 10 [g/g] to 35 [g/g], morepreferably in a range of 10 [g/g] to 32 [g/g], and further morepreferably in a range of 15 [g/g] to [g/g]. Further, the gel CRC afterthe gel grinding is increased by preferably −1 [g/g] to +3 [g/g] thangel CRC before the gel grinding, more preferably 0.1 [g/g] to 2 [g/g],and further more preferably 0.3 [g/g] to 1.5 [g/g]. Note that the gelCRC after the gel grinding can be decreased during the gel grinding byuse of a crosslinking agent, or the like. It is, however, preferablethat the gel CRC after the gel grinding be increased in the above range.

(c) Gel Ext After Gel Grinding

In the present invention, gel Ext of the particulate hydrogel after thegel grinding is preferably in a range of 0.1 wt % to 20 wt %, morepreferably in a range of 0.1 wt % to 10 wt %, further more preferably ina range of 0.1 wt % to 8 wt %, and especially preferably in a range of0.1 wt % to 5 wt %. Further, how much the gel Ext of the particulatehydrogel is increased by the gel grinding (i.e. an increase in theamount of the gel Ext with respect to gel Ext before the gel grinding)is preferably not more than 5 wt %, more preferably not more than 4 wt%, further more preferably not more than 3 wt %, especially preferablynot more than 2 wt %, and most preferably not more than 1 wt %. Theincrease in the amount of the gel Ext of the particulate hydrogel by thegel grinding also can have a minus lower limit (for example, −3.0 wt %,further −1.0 wt %). Normally, however, the increase in the amount of thegel Ext of the particulate hydrogel by the gel grinding is not less than0 wt %, preferably not less than 0.1 wt %, more preferably not less than0.2 wt %, and further more preferably not less than 0.3 wt %.Specifically, the hydrogel is ground so that the gel Ext is increased inan arbitrary range of the upper limits and the lower limits such aspreferably in a range of 0 to 5.0 wt %, and more preferably in a rangeof 0.1 wt % to 3.0 wt %. Note that the gel Ext can be decreased duringthe gel grinding by use of a crosslinking agent, or the like. It is,however, preferable that the gel Ext be increased in the above range.Note here that an effective digit of the amount of the gel Ext toincrease is the first decimal place, and for example, 5 wt % is regardedas a synonym for 5.0 wt %.

(d) Weight Average Molecular Weight of Water Soluble Content After GelGrinding

In the present invention, an amount of weight average molecular weightof water soluble content of the hydrogel to be increased by the gelgrinding has (i) a lower limit of preferably not less than 10,000 [Da],more preferably not less than 20,000 [Da], and further more preferablynot less than 30,000 [Da], and (ii) an upper limit of preferably notmore than 500,000 [Da], more preferably not more than 400,000 [Da],further more preferably not more than 250,000 [Da], and especiallypreferably not more than 100,000 [Da]. Therefore, in the presentinvention, an increase in weight average molecular weight of watersoluble content of the particulate hydrogel after the gel grinding withrespect to the hydrogel before the gel grinding is, for example, in arange of 10,000 [Da] to 500,000 [Da], preferably in a range of 20,000[Da] to 400,000 [Da], more preferably in a range of 30,000 [Da] to250,000 [Da], and further more preferably not more than 100,000 [Da].

In conventional well-known gel grinding, an increase in weight averagemolecular weight of water soluble content is frequently less than 10,000[Da]. Meanwhile, the present invention is characterized in that a mainchain part of a polymer of the hydrogel is cut by a greater gel grindingenergy (GGE), that is, greater shearing stress and compressive force sothat the weight average molecular weight of the water soluble content isincreased. Note, however, that it is not preferable that the amount ofthe weight average molecular weight of the water soluble content to beincreased by the gel grinding be greater than 500,000 [Da] because, ifso, a crosslinked polymer chain of the hydrogel is cut by excessivemechanical external force so that the water soluble content isexcessively increased, whereby the properties of the hydrogel aredeteriorated.

(e) Resin Solid Content After Gel Grinding

In the present invention, resin solid content of the particulatehydrogel after the gel grinding is, in terms of its property, preferablyin a range of 10 wt % to 80 wt %, more preferably in a range of 30 wt %to 80 wt %, further more preferably in a range of 50 wt % to 80 wt %, 45wt % to 85 wt %, or 45 wt % to 70 wt %, and especially preferably in arange of 50 wt % to 60 wt % or 45 wt % to 60 wt %. It is preferable thatthe resin solid content of the particulate hydrogel after the gelgrinding have the above range because in the range, increase in the CRCdue to drying is easily controllable, and damage caused by drying isdecreased (for example, the water soluble content is less increased).Note that the resin solid content after the gel grinding can beappropriately controlled by, for example, resin solid content before thegel grinding, if necessary, water to be added, or water vaporization byheating during the gel grinding.

(Pieces to be Measured)

The properties of the hydrogel before the gel grinding, or theproperties of the particulate hydrogel after the gel grinding, areestimated by sampling and measuring, at a necessary frequency, anecessary amount of the hydrogel or the particulate hydrogel that is ina production device. In the present invention, the estimation is carriedout on the basis of the weight average molecular weight of the watersoluble content of the hydrogel before the gel grinding. A value of theweight average molecular weight of the water soluble content of thehydrogel before the gel grinding should be a numeric value that issufficiently averaged. In order to calculate the numeric value, forexample, the following sampling and measurement are carried out. In acase where the water absorbent resin powder is produced by 1 [t/hr] to20 [t/hr], or 1 [t/hr] to 10 [t/hr] by use of a continuous gel grindingdevice such as a continuous kneader or a meat chopper, two or morepieces per 100 kg of the hydrogel, ten or more pieces in total aresampled and measured. In a case of batch-type gel grinding (such as abatch-type kneader), ten or more pieces are sampled from a batch sample,and measured. The properties of the particulate hydrogel are estimatedon the basis of the sampling and the measurement.

(2-3) Drying Step (First through Fourth Producing Methods of the PresentInvention)

The drying step is a step for drying the particulate hydrogel obtainedin the gel grinding step to obtain a dried polymer. The followingdescribes a drying method suitably applicable to the present invention.The drying method is applicable to the first through fourth producingmethods of the present invention. Among these producing methods, thefourth producing method employs specific dry temperature and hot windvelocity. Such specific dry temperature and hot wind velocity aresuitably applicable to the first through third producing methods, andcontribute to improvement of the water absorbing rate.

Examples of the drying method in the drying step of the presentinvention encompass thermal drying, hot air drying, drying under reducedpressure, infrared drying, microwave drying, drying by use of a drumdrier, drying by azeotropic dehydration with a hydrophobic organicsolvent, and high humidity drying with use of high temperature watervapor. Among the drying methods, the hot air drying, particularly with adew point of 40° C. to 100° C., more preferably a dew point of 50° C. to90° C., is preferably adopted.

In a further preferred embodiment, a belt drier is employed as a drierto be used in the drying step. If necessary, one or more types of a heattransfer drier, a radiation heating drier, a hot air drier, a dielectricheating drier, and like driers can be used in combination with the beltdrier. Among the driers, the hot air drier is preferably used in termsof a drying rate. Examples of the hot air drier encompass a through-flowbelt (band) hot air drier, a through-flow circuit hot air drier, avertical through-flow hot air drier, a parallel through-flow belt (band)hot air drier, a through-flow tunnel hot air drier, a through-flowgroove stirring hot air drier, a fluidized-bed hot air drier, a flashhot air drier, and a spray hot air drier. Among the hot air driers, thethrough-flow belt hot air drier is preferably used in terms of propertycontrol in the present invention.

In the present invention, the particulate hydrogel can be dried by theabove-described various driers. Among the driers, the through-flow beltdrier, especially the through-flow belt hot air drier is preferably usedin the first through fourth producing methods of the present invention.In a case where the through-flow belt hot air drier is used, thethrough-flow belt hot air drier should send hot air to a hydrogel layerdisposed on a through-flow belt from a direction vertical to thehydrogel layer (for example, from both above and blow the hydrogellayer, from below the hydrogel layer, or from above the hydrogel layer).In a case where the through-flow belt hot air drier is not used or in acase where hot air is not sent from the direction vertical to thehydrogel layer, it is impossible to uniformly dry the hydrogel layer.This possibly deteriorates the properties such as the permeabilitypotential of the particulate hydrogel. That is, neither hot air from alateral direction nor other driers (such as a fluidized-bed drier or astirring drier) can attain the object of the present invention. Notethat what is meant by the “direction vertical to the hydrogel layer” isa state in which hot air flows through a gel layer (a layer ofparticulate hydrogel having a thickness in a range of 10 mm to 300 mm ona punching metal or a woven metallic wire) in an up-and-down directionto the gel layer (from above the gel layer to below the gel layer, orfrom below the gel layer to above the gel layer). The direction verticalto the gel layer is not limited to a completely vertical direction aslong as hot air flows through the gel layer in the direction vertical tothe gel layer. Therefore, hot air can be sent from an oblique direction,for example, hot air is sent from a direction within 30° from thevertical direction, preferably within 20°, more preferably within 10°,further preferably within 5°, and especially preferably from thecompletely vertical direction.

The following describes drying conditions etc. of the drying step of thepresent invention. By drying the particulate hydrogel under the dryingconditions, it is possible to improve permeability potential and a waterabsorbing rate of water absorbent resin powder in which a dried polymerattained by drying the particulate hydrogel under the drying conditionsis subjected to a surface treatment.

(Drying Temperature)

Drying temperature in the drying step (preferably in the through-flowbelt drier) of the present invention is in a range of 100° C. to 300°C., preferably in a range of 150° C. to 250° C., more preferably in arange of 160° C. to 220° C., and further preferably in a range of 170°C. to 200° C. A drying temperature in the range of 100° C. to 300° C.makes it possible to reduce drying time and coloring of the driedpolymer. Further, such a drying temperature brought an effect ofimproving the permeability potential and the water absorbing rate of thewater absorbent resin powder. Meanwhile, a drying temperature of morethan 300° C. causes a polymer chain to be damaged, thereby deterioratingthe properties of the water absorbent resin powder. Moreover, a dryingtemperature of less than 100° C. does not bring the effect of improvingthe water absorbing rate, and causes (i) failure of drying resulting inundried particulate hydrogel, and (ii) clogging during a subsequentgrinding step.

(Drying Time)

Drying time in the drying step (preferably of the through-flow beltdrier) of the present invention depends on a surface area of theparticulate hydrogel, types of a drier, and the like, and may beappropriately determined so that an objective water content is attained.However, the drying time is preferably in a range of 1 minute to 10hours, more preferably in a range of 5 minutes to 2 hours, furtherpreferably in a range of 10 minutes to 120 minutes, and especiallypreferably in a range of 20 minutes to 60 minutes.

A period of time that elapses before the particulate hydrogel dischargedfrom the gel grinding step of (2-2) proceeds to the drying step, thatis, a period of time of moving of the particulate hydrogel from anoutlet of the gel grinding device to an inlet of the drier is preferablyshorter in terms of coloring of the dried polymer. Specifically, theperiod of time is preferably within 2 hours, more preferably within 1hour, further preferably within 30 minutes, especially preferably with10 minutes, and most preferably within 2 minutes.

(Wind Velocity)

In order to attain the object of the present invention, the through-flowdrier, especially the belt drier sends hot air in the vertical direction(up-and-down direction) at a wind velocity of 0.8 [m/s] to 2.5 [m/s],preferably 1.0 [m/s] to 2.0 [m/s]. The wind velocity in the above rangemakes it possible not only to control water content of the dried polymerto be in a desired range but also to improve the water absorbing rate.It was found that a wind velocity of less than 0.8 [m/s] results inextension of the drying time, thereby deteriorating the permeabilitypotential and the water absorbing rate of the water absorbent resinpowder. It was also found that a wind velocity of more than 2.5 [m/s]causes the particulate hydrogel to be blown up during drying, therebymaking it difficult to stably dry the particulate hydrogel.

Note that the wind velocity is controlled so as not to impair the effectof the present invention, and therefore the wind velocity may becontrolled as above, for example, during 70% or more of the drying time,preferably 90% or more, and further preferably 95% or more. Note alsothat in a case of the through-flow belt drier, the wind velocityrepresents an average flow rate of hot air passing through in adirection vertical to a surface of the through-flow belt thathorizontally moves. Therefore, the average flow rate of hot air iscalculated by dividing, by a surface area of the through-flow belt,quantity of hot air sent by the through-flow belt drier.

It was found that it is possible to improve the permeability potentialand the water absorbing rate of the water absorbent resin powder bydrying, by use of the through-flow belt hot air drier for drying atspecific temperature and wind velocity, the particulate hydrogel, with aspecific particle diameter, which has been obtained in the gel grindingstep. That is, the wind velocity in the above range makes it possible toimprove the water absorbing rate of the dried polymer.

Note that as to a gel particle diameter and drying of water absorbentresin, Patent Literature 50 discloses a technique of drying aparticulate (angular) hydrogel polymer which is subjected to gelgrinding to have an average particle diameter in a range of 0.8 mm to 5mm, preferably in a range of 1 mm to 3 mm, and logarithmic standarddeviation (σζ) of particle size distribution of 1.5 or less, preferably0.8 or less, in terms of reduction in water content, a dryingefficiency, etc.

However, the technique disclosed in Patent Literature 50 is a techniqueof controlling polyhedral (angular) hydrogel having flat surfaces by useof a vertical cutter. This is different from the gel grinding of thepresent invention. Patent Literature 50 neither discloses nor suggests awater absorbing rate (FSR) and permeability potential (SFC) of the waterabsorbent resin, the specific wind velocity (0.8 [m/s] to 2.5 [m/s])during drying, and a specific surface crosslinking (particularly,combination use of an ionic bonding crosslinking agent) (laterdescribed). That is, it was found in the present invention that thespecific wind velocity during drying and the specific surfacecrosslinking which are not disclosed in Patent Literature 50, andfurther the gel grinding energy (GGE or GGE (2)), and the increase [Da]in the weight average molecular weight of the gel Ext greatly affect thewater absorbing rate (FSR) and the permeability potential (SFC) of thewater absorbent resin.

(Dew Point of Hot Air)

The hot air sent by the through-flow belt drier in the drying step ofthe present invention contains at least water vapor, and has a dew pointof preferably 30° C. to 100° C., and more preferably 30° C. to 80° C.Controlling the dew point in the above range, and further preferably thegel particle diameter in the above range make it possible to reduceresidual monomers, and further prevent reduction in bulk specificgravity of the dried polymer. Note that the dew point is a value wherethe particulate hydrogel has water content of 10 wt % or more,preferably 20 wt % or more.

Further, it is preferable that a dew point in the vicinity of the inletof the drier (or in the early period of drying, for example, within 50%of the drying time) be higher than that in the vicinity of an outlet ofthe drier (or in the last period of the drying, for example, over 50% ofthe drying time) in terms of residual monomers, absorbency, coloring,etc. Specifically, it is preferable to expose the particulate hydrogelto hot air having a dew point higher by preferably 10° C. to 50° C.,more preferably 15° C. to 40° C. Controlling the dew point in the aboverange makes it possible to prevent the reduction in the bulk specificgravity of the dried polymer.

In the drying step of the present invention, the particulate hydrogel iscontinuously supplied so as to form a layer on the belt of thethrough-flow belt drier, and then dried by hot air. A width of the beltof the through-flow belt drier is not limited to a specific one but ispreferably 0.5 m or more, and more preferably 1 m or more. The width ofthe belt also has an upper limit of preferably 10 m or less, and morepreferably 5 m or less. Further, a length of the belt is preferably 20 mor more, and more preferably 40 m or more. The length of the belt alsohas an upper limit of preferably 100 m or less, and more preferably 50 mor less.

In order to attain the object of the present invention, a layer length(a thickness of a gel layer) of the particulate hydrogel on the belt ispreferably in a range of 10 mm to 300 mm, more preferably in a range of50 mm to 200 mm, further preferably in a range of 80 mm to 150 mm, andespecially preferably in a range of 90 mm to 110 mm.

A moving rate of the particulate hydrogel on the belt may beappropriately determined on the basis of the width of the belt, thelength of the belt, production quantity, the drying time, etc. In termsof load on a belt driving device, endurance of the belt driving device,etc., however, the moving rate is preferably in a range of 0.3 [m/min]to 5 [m/min], more preferably in a range of 0.5 [m/min] to 2.5 [m/min],further preferably in a range of 0.5 [m/min] to 2 [m/min], andespecially preferably in a range of 0.7 [m/min] to 1.5 [m/min].

The method for producing the water absorbent polyacrylic acid (salt)resin powder of the present invention is suitable for continuousproduction, and brings a great effect of improving productivity, theproperties of the water absorbent resin powder, etc. by settingconditions to the respective ranges in the drying step.

The through-flow belt drier used in the drying step of the presentinvention preferably has the following specified structure andconfiguration. Examples of the through-flow belt encompass a wovenmetallic wire (for example, having a mesh size of 45 μm to 1000 μm), anda punching metal. Among these, the punching metal is preferablyemployed. A shape of holes of the punching metal is not particularlylimited. Examples of the shape of holes encompass a circular hole, anelliptic hole, an angular hole, a hexagonal hole, an oblong hole, arectangular hole, a rhombic hole, a cross-shaped hole, and combinationof the plurality of shapes of holes. Further, the holes can be arrangedin a staggered manner or in a juxtaposition manner. The holes each canbe a three-dimensional hole such as a louver (a bow window). It is,however, preferable that the hole be a flat hole. Moreover, a pitchdirection of the holes can be a direction longitudinal, lateral, oroblique to a direction in which the belt extends, or a combination ofthe directions.

In order to attain the present invention, it is preferable to variouslychange the drying temperature, the dew point of hot air, and the airquantity. It is therefore preferable that the through-flow belt drierinclude preferably five or more rooms, more preferably six or morerooms, and further preferably eight or more rooms. An upper limit of thenumber of rooms is appropriately determined depending on productionquantity, the size of the through-flow belt drier, etc. Normally,however, the upper limit is approximately 20.

(Resin Solid Content)

The particulate hydrogel obtained in the gel grinding step is dried inthe drying step to be the dried polymer. Resin solid content calculatedfrom drying loss of the dried polymer (heating 1 g of powder orparticles at 180° C. for three hours) is preferably more than 80 wt %,more preferably in a range of 85 wt % to 99 wt %, further preferably ina range of 90 wt % to 98 wt %, and especially preferably in a range of92 wt % to 97 wt %.

(Surface Temperature of Particulate Hydrogel)

The particulate hydrogel obtained in the gel grinding step has a surfacetemperature preferably in a range of 40° C. to 110° C., more preferablyin a range of 60° C. to 110° C., further preferably in a range of 60° C.to 100° C., and especially preferably in a range of 70° C. to 100° C.,immediately before being supplied into the through-flow belt drier. Asurface temperature of less than 40° C. will generate a balloon-likedried polymer during drying, and plenty of fine powder during grinding,thereby deteriorating the properties of the dried polymer. Moreover, asurface temperature of more than 110° C. will cause propertydeterioration (such as increase in water soluble content) and coloringof the water absorbent resin after drying.

(2-4) Grinding Step and Classifying Step

The grinding step and the classifying step are a step for grinding andclassifying the dried polymer obtained in the drying step to obtainwater absorbent resin particles. Note that the grinding step isdifferent from (2-2) Gel Grinding Step in resin solid content duringgrinding (grinding), especially in that a target to be ground in thegrinding step has been dried (preferably the resin solid content hasbeen dried). The water absorbent resin particles obtained after thegrinding step may be called a ground dried polymer.

The dried polymer obtained in the drying step can be used as it is aswater absorbent resin powder. It is, however, preferable to control thedried polymer to have a specific particle diameter so as to improve theproperties in a surface treatment step, particularly in a surfacecrosslinking step (later described). The particle diameter of the driedpolymer can be appropriately controlled not only in the grinding step orthe classifying step but also in the polymerization step, a fine powdercollecting step, a granulation step, or like step. The particle diameteris defined by a standard sieve (JIS Z8801-1 (2000)).

Any grinders can be used in the grinding step. Examples of the grindersencompass a vibration mill, a roll granulator, a knuckle grinder, a rollmill, a high-speed grinder (such as a pin mill, a hammer mill, and ascrew mill), and a cylindrical mixer. Among the grinders, it ispreferable to use a multiple-stage roll mill or roll granulator in termsof particle diameter control.

The classifying step is carried out so that the water absorbent resinparticles have the following particle diameter. In a case where surfacecrosslinking is carried out, it is preferable to carry out theclassifying step before the surface crosslinking step (first classifyingstep). The classifying step may be further carried out after the surfacecrosslinking step (second classifying step). Note that how to carry outthe classifying step is not particularly limited. For example, theclassifying step is carried out by use of a sieve as below. In a casewhere particle size distribution of the water absorbent resin particlesis set to 150 μm to 850 μm, first, the ground dried polymer is sieved byuse of a sieve having a mesh size of 850 μm, and then a ground driedpolymer that has passed through the sieve is further sieved by use of asieve having a mesh size of 150 μm or a sieve having a mesh size of morethan 150 μm (for example, 200 μm). A ground dried polymer left on thesieve having the mesh size of 150 μm is the water absorbent resinparticles having a desired particle size distribution. The classifyingstep can be carried out not only by classification by sieving but alsoby various classifiers such as classification by air.

The water absorbent resin particles have, after the classifying step, aweight average particle diameter (D50) preferably in a range of 250 μmto 500 μm, more preferably in a range of 300 μm to 500 μm, and furtherpreferably in a range of 350 μm to 450 μm, in terms of improvement ofthe properties of the water absorbent resin powder to be produced in thepresent invention. Further, it is preferable that an amount of fineparticles passing through the sieve having the mesh size of 150 μm (JISstandard sieve) be smaller. Specifically, fine particles that passthrough the sieve are normally preferably in a range of 0 to 5 wt % withrespect to the whole water absorbent resin particles, more preferably ina range of 0 to 3 wt %, and further preferably in a range of 0 to 1 wt%. It is also preferable that as less as possible of large particlespass through a sieve having a mesh size of 850 μm or more (or 710 μm ormore) (JIS standard sieve). Specifically, large particles that passthrough the sieve are normally preferably in a range of 0 to 5 wt % withrespect to the whole water absorbent resin particles, more preferably ina range of 0 to 3 wt %, and further preferably in a range of 0 to 1 wt%. In the present invention, a ratio of particles whose particlediameter is not less than 150 μm and less than 850 μm to the whole waterabsorbent resin particles, further a ratio of particles whose particlediameter is not less than 150 μm and less than 710 μm to the whole waterabsorbent resin particles is adjusted to preferably 95 wt % or more, andmore preferably 98 wt % or more (an upper limit is 100 wt %).Logarithmic standard deviation (σζ) of the particle size distribution ispreferably in a range of 0.20 to 0.50, more preferably in a range of0.25 to 0.50, further preferably in a range of 0.25 to 0.45, andespecially preferably in a range of 0.30 to 0.40. The particle diameteris measured by a method similar to the method described in “(1) AverageParticle Diameter and Distribution of Particle Diameter” (see page 7,lines 25 to 43 in the specification of EP Patent No. 0349240). Anadditional standard sieve (mesh size) may be appropriately used forparticle diameter measurement depending on a particle diameter of atarget to be measured. For example, a standard sieve having a mesh sizeof, for example, 710 μm or 600 μm is additionally used. The particlediameter before the surface crosslinking is applied to preferably afterthe surface crosslinking, and further an end product.

(Internal Cell Ratio)

The water absorbent resin powder attained by the gel grinding of thepresent invention, and further preferably by drying at the specifictemperature and wind velocity can have a specific internal cell ratio.The internal cell ratio of the water absorbent resin powder, and apreferred range of the internal cell ratio will be described in [3], andare applied to the water absorbent resin particles obtained in thegrinding step and the classifying step. That is, the water absorbentresin particles before the surface crosslinking preferably have (i) notless than 95 wt % of particles whose particle diameter is not less than150 μm and less than 850 μm, (ii) the logarithmic standard deviation(σζ) of the particle size distribution in the range of 0.25 to 0.50, and(iii) the internal cell ratio (defined by the following expression)preferably in a range of 0.1% to 2.5%, more preferably in a range of0.2% to 2.0%, further preferably in a range of 0.3% to 1.7%, andespecially preferably in a range of 0.5% to 1.5%. It is possible toprovide the water absorbent resin powder whose water absorbing rate(FSR) and permeability potential (SFC) are both attained, by subjectingthe water absorbent resin particles having the internal cell ratio andthe particle size distribution to surface crosslinking particularly soas to have absorption against pressure (AAP) of not less than 20 [g/g].Thus, the object of the present invention is further attained.(Internal cell ratio) [%]={(real density)−(apparent density)}/(realdensity)×100

Note that the water absorbent resin before the surface crosslinking isnot limited to having the internal cell ratio and the particle sizedistribution. The following describes the surface crosslinking of thepresent invention.

(2-5) Surface Treatment Step

The method for producing the water absorbent polyacrylic acid (salt)resin powder of the present invention preferably further includes asurface treatment step in order to improve the absorbency (absorbencyagainst pressure, permeability potential, water absorbing rate, etc.).The surface treatment step includes a surface crosslinking stepperformed by use of a conventional surface crosslinking agent by aconventional surface crosslinking method, and if necessary, furtherincludes an addition step.

(Covalent Bonding Surface Crosslinking Agent)

Various organic or inorganic crosslinking agents can be exemplified asthe surface crosslinking agent for use in the present invention, but itis preferable that the surface crosslinking agent be an organic surfacecrosslinking agent. For the sake of the properties, it is preferable touse a dehydrative crosslinking agent such as (i) a polyvalent alcoholcompound, (ii) an epoxy compound, (iii) a condensed product with apolyvalent amine compound or a halo epoxy compound, (iv) an oxazolinecompound, (v) a (mono, di, or poly)oxazolidinone compound, and (vi) analkylene carbonate compound. Especially, a dehydrative crosslinkingagent such as a polyvalent alcohol compound, an alkylene carbonatecompound, or an oxazolidinone compound, which needs high-temperaturereaction can be used. In a case where a dehydrative crosslinking agentis not used, more specifically, the compounds described in U.S. Pat.Nos. 6,228,930, 6,071,976, 6,254,990, etc. can be exemplified. Forexample, polyvalent alcohols, such as mono-, di-, tri-, tetra-propyleneglycol, 1,3-propanediol, glycerin, 1,4-butanediol, 1,3-butanediol,1,5-pentanediol, 1,6-hexanediol, and sorbitol; epoxy compounds, such asethylene glycol diglycidyl ether, and glycidol; alkylene carbonatecompounds such as ethylene carbonate; oxetane compounds; cyclic ureacompounds, such as 2-imidazolidinone are exemplified.

(Solvent, etc.)

An amount of the surface crosslinking agent to use is determined asappropriate, preferably in a range of 0.001 parts by weight to 10 partsby weight, and more preferably in a range of 0.01 parts by weight to 5parts by weight with respect to 100 parts by weight of the waterabsorbent resin particles. In addition to the surface crosslinkingagent, water is used in combination preferably. An amount of the waterused herein is preferably in a range of 0.5 parts by weight to 20 partsby weight, and more preferably in a range of 0.5 parts by weight to 10parts by weight with respect to 100 parts by weight of the waterabsorbent resin particles. In case where an inorganic surfacecrosslinking agent and an organic surface crosslinking agent are used incombination, the surface crosslinking agents are respectively used in anamount preferably in a range of 0.001 parts by weight to 10 parts byweight, and more preferably in a range of 0.01 parts by weight to 5parts by weight with respect to 100 parts by weight of the waterabsorbent resin particles.

In this case, a hydrophilic organic solvent may be used in an amountpreferably in a range of 0 to 10 parts by weight, more preferably in arange of 0 to 5 parts by weight, with respect to 100 parts by weight ofthe water absorbent resin particles. In adding a surface crosslinkingagent solution to the water absorbent resin particles, water insolublefine particle powder or an interfacial active agent may be added as wellin an amount not adversely affecting the effect of the presentinvention, for example, in a range of 0 to 10 parts by weight,preferably in a range of 0 to 5 parts by weight, and more preferably ina range of 0 to 1 part by weight. Examples of usable interfacial activeagents and an amount of the interfacial active agent to use areexemplified in U.S. Pat. No. 7,473,739 B.

(Mixing)

In a case where the surface crosslinking agent solution is mixed withthe water absorbent resin particles, the water absorbent resin particlesswell with, for example, water of the surface crosslinking agentsolution. Swollen water absorbent resin particles are dried by heat at apreferred temperature of 80° C. to 220° C. for a preferred time of 10minutes to 120 minutes.

The mixing of the surface crosslinking agent with the water absorbentresin particles is carried out preferably by using a vertical orhorizontal high-speed rotation stirring mixer. The number of revolutionsof the mixer is preferably in a range of 100 rpm to 10000 rpm, and morepreferably in a range of 300 rpm to 2000 rpm. Further, a period of timefor which the surface crosslinking agent and the water absorbent resinparticles are retained in the mixing by the mixer is preferably within180 seconds, more preferably in a range of 0.1 to 60 seconds, andfurther more preferably in a range of 1 to 30 seconds.

(Other Surface Crosslinking Method)

A surface crosslinking method employing a radical polymerizationinitiator (see U.S. Pat. No. 4,783,510, and PCT InternationalPublication No. 2006/062258), and a surface crosslinking method in whicha monomer(s) is polymerized on a surface of water absorbent resin (seeUS Patent Application Publications Nos. 2005/048221 and 2009/0239966,and PCT International Publication No. 2009/048160) can be substitutedfor a surface crosslinking method of the present invention employing thesurface crosslinking agent.

In the surface crosslinking method of the present invention, a preferredexample of the radical polymerization initiator to use is persulfate, apreferred example of the monomer(s) to be arbitrarily used is acrylicacid (salt) or the above-described crosslinking agents, and a preferredexample of a solvent to use is water. The materials are added onto thesurface of the water absorbent resin, and then by an active energy line(particularly ultraviolet ray) or heat, crosslinking polymerization isperformed on the surface of the water absorbent resin, or crosslinkingreaction is caused by use of the radical polymerization initiator. Thus,the surface crosslinking is attained.

(Ionic Bonding Surface Crosslinking Agent)

The method for producing the water absorbent polyacrylic acid (salt)resin powder of the present invention further includes an addition stepfor adding at least one of a polyvalent metal salt, a cationic polymer,and inorganic fine particles. The addition step is carried outconcurrently or non-concurrently with the surface crosslinking step.That is, the permeability potential, the water absorbing rate, etc. maybe improved by solely using the inorganic surface crosslinking agent orby using the inorganic surface crosslinking agent together with theorganic surface crosslinking agent in combination. The inorganic surfacecrosslinking agent and the organic surface crosslinking agent may beadded concurrently or separately. Examples of the inorganic surfacecrosslinking agent to use encompass divalent or greater, preferablytrivalent or tetravalent metal salt (organic salt or inorganic salt),and hydroxide. Usable polyvalent metals include aluminum and zirconium.Aluminum lactate and aluminum sulfate are also exemplified. An aqueoussolution containing aluminum sulfate is preferably employed. Theinorganic surface crosslinking agent is used concurrently ornon-concurrently with the organic surface crosslinking agent. Thesurface crosslinking with the use of the polyvalent metals is disclosedin PCT International Publications Nos. 2007/121037, 2008/09843, and2008/09842, U.S. Pat. Nos. 7,157,141, 6,605,673, and 6,620,889, and USPatent Application Publications Nos. 2005/0288182, 2005/0070671,2007/0106013, and 2006/0073969.

Moreover, the permeability potential etc. may be improved by asimultaneous or separate use of a cationic polymer particularly havingweight average molecular weight of approximately 5,000 to 1,000,000. Apreferred example of the cationic polymer to use is a vinyl aminepolymer (see U.S. Pat. No. 7,098,284, PCT International PublicationsNos. 2006/082188, 2006/082189, 2006/082197, 2006/111402, 2006/111403,and 2006/111404, etc.).

Further, the inorganic fine particles may be used. A preferred exampleof the inorganic fine particles is silicon dioxide (see U.S. Pat. No.7,638,570 etc.).

A preferred production method in the present invention is the method forproducing the water absorbent resin including the addition step foradding at least one of the polyvalent metal salt, the cationic polymer,and the inorganic fine particles. Such additives are preferably addedconcurrently or separately with the covalent bonding surfacecrosslinking agent. This makes it possible to further attain the object(the permeability potential and the water absorbing rate) of the presentinvention.

(Property After Surface Crosslinking)

The water absorbent resin powder obtainable by the gel grinding of thepresent invention, and further preferably by drying at the specifictemperature and wind velocity can have the specific internal cell ratio.The present invention is not limited to the water absorbent resinpowder. It is, however, preferable in the present invention that thesurface crosslinking is performed with reaction temperature, reactiontime, etc. appropriately adjusted so that the absorption againstpressure (AAP) becomes not less than 20 [g/g], further a range describedin (3-1) (later described) after the surface crosslinking, and theabsorbency without pressure (CRC) becomes a range described in (3-3)(later described) after the surface crosslinking.

It is possible to provide the water absorbent resin powder in which thewater absorbing rate (FSR) and the permeability potential (SFC) are bothattained, by the gel grindings (the first to fourth producing methods)of the present invention, and further preferably by the particlediameter control of the dried polymer and the surface crosslinking.Thus, the object of the present invention is further attained. Note thatnovel water absorbent resin powder of the present invention will bedescribed in detail in [3], the novel water absorbent resin powder beingproduced by, for example, the above-described production method.

Note also that in the production method of the present invention, thewater absorbent resin before the surface crosslinking is not limited tohaving the internal cell ratio and the particulate diameterdistribution.

(2-6) Other Steps (Fine Powder Recycling Step etc.)

Besides those steps described above, an evaporated monomer recyclingstep, a granulation step, a fine powder removing step, a fine powderrecycling step, or like step may be provided, if necessary. Further, ifnecessary, the following additive may be added to some or all of thesteps, in order to attain color stability over time or prevent gelproperty deterioration, etc. That is, a water-soluble or water-insolublepolymer, a lubricant, a chelating agent, deodorant, an antimicrobialagent, water, an interfacial active agent, water-insoluble fineparticles, anti-oxidant, a reducing agent, or the like can be mixed withthe water absorbent resin in an amount of preferably 0 wt % to 30 wt %,and more preferably 0.01 wt % to 10 wt %. Such additives can be used asa surface treatment agent.

The production method of the present invention can include the finepowder recycling step. What is meant by the fine powder recycling stepis a step for separating fine powder (particularly fine powdercontaining not less than 70 wt % of fine particles each having aparticle diameter of not more than 150 μm) generated in the drying step,and further in the grinding step and in the classifying step (if thegrinding step and the classifying step are performed), and thenrecycling, in the polymerization step or in the drying step, the finepowder as it is or after being hydrated. The methods disclosed in USPatent Application Publication No. 2006/247351, U.S. Pat. No. 6,228,930,etc. can be applied to the fine powder recycling step of the presentinvention.

Furthermore, if required by the purpose thereof, the water absorbentresin may contain an oxidant, an anti-oxidant, water, a polyvalent metalcompound, water-insoluble inorganic or organic powder such as silica ormetal soap, deodorant, an antimicrobial agent, polymer polyamine, orpulps, thermoplastic fiber, in an amount of 0 wt % to 3 wt %, preferably0 wt % to 1 wt %.

(2-7) Summary

In other words, a method for producing water absorbent resin powder ofthe present invention (first to fourth producing methods) is a methodfor producing water absorbent polyacrylic acid (salt) resin powder,including the steps of: (i) polymerizing an acrylic acid (salt) monomeraqueous solution; (ii) during or after the step of (i), performing gelgrinding of a hydrogel crosslinked polymer obtained by thepolymerization, wherein the hydrogel crosslinked polymer has resin solidcontent of 10 wt % to 80 wt %; (iii) drying a particulate hydrogelcrosslinked polymer obtained by the gel grinding, wherein the drying isperformed at 150° C. to 250° C.; and (iv) carrying out a surfacetreatment to the particulate hydrogel crosslinked polymer thus dried,the step of (ii) being carried out such that at least one of (1) to (4)is met, where (1) the gel grinding is carried out with gel grindingenergy (GGE) of 18 [J/g] to 60 [J/g]; (2) the gel grinding is carriedout with gel grinding energy (2) (GGE (2)) of 9 [J/g] to 40 [J/g]; (3)weight average molecular weight of water soluble content of the hydrogelcrosslinked polymer is increased by 10,000 [Da] to 500,000 [Da]; and (4)the particulate hydrogel crosslinked polymer obtained by the step of(ii) has a weight average particle diameter (D50) of 350 μm to 2000 μm,and logarithmic standard deviation (σζ) of particle size distribution of0.2 to 1.0.

In a case where the hydrogel crosslinked polymer is subjected to the gelgrinding so that (4) is met, the particulate hydrogel crosslinkedpolymer is dried by a through-flow (belt) drier, the particulatehydrogel crosslinked polymer to be supplied into the through-flow (belt)drier has resin solid content of 10 wt % to 80 wt %, and thethrough-flow belt drier sends hot air of 150° C. to 250° C. at avelocity of 0.8 [m/s] to 2.5 [m/s] in a direction vertical (up-and-downdirection) to the particulate hydrogel crosslinked polymer.

The gel grinding of the present invention essentially meets at least oneof the gel grindings (1) to (4), preferably two or more, furtherpreferably three or more, and especially preferably all of them.Further, it is preferable that not only the particulate hydrogelcrosslinked polymer obtained by the gel grinding of (4) but also theparticulate hydrogel crosslinked polymer obtained by the gel grinding of(1) to (3) be dried by the through-flow belt drier under the dryingcondition (such as the hot air velocity). Moreover, it is furtherpreferable that surface crosslinking be performed especially bycombination use of a covalent bonding surface crosslinking agent and anionic bonding surface crosslinking agent.

Permeability potential (SFC) and a water absorbing rate (FSR) of waterabsorbent resin greatly depend on a surface area of the water absorbentresin, and are inversely proportional to each other. That is, the SFC ishigher as the surface area is smaller, and in contrast, the FSR ishigher as the surface area is larger. Therefore, conventional techniqueshave difficulty in attaining both the SFC and the FSR.

Meanwhile, according to the production method of the present invention,it is possible to attain the following ranges of the FSR and the SFC,especially (i) FSR of not less than 0.30 [g/g/sec], further a high waterabsorbing rate which will be described in (3-6), and (ii) SFC of notless than 70 [×10⁻⁷·cm³·s·g⁻¹], further a high permeability potentialwhich will be described in (3-2). The production method of the presentinvention is suitably applicable to a method for producing waterabsorbent resin having such high FSR and SFC. Note that preferredproperties will be described in [3].

Patent Literatures 1 through 50, etc. are conventionally known astechniques for improving the water absorbing rate, the permeabilitypotential, etc of the water absorbent resin. Meanwhile, it was found inthe present invention that it is possible to improve and attain thewater absorbing rate (such as the FSR) and the permeability potential(such as the SFC) by at least one of the specific gel grindings (1) to(4).

[3] Property of Water Absorbent Polyacrylic Acid (Salt) Resin

(Novel Water Absorbent Resin)

The water absorbent polyacrylic acid (salt) resin produced by theproduction method of the present invention (the first to fourthproducing methods) that is a preferred production method, has (i) notless than 95 wt % of the particles having the particle diameter of notless than 150 μm and less than 850 μm, (ii) the logarithmic standarddeviation (σζ) of the particle size distribution in the range of 0.25 to0.50, (iii) the absorption against pressure (AAP) of not less than 20[g/g], (iv) the water absorbing rate (FSR) of not less than 0.30[g/g/s], and (v) the internal cell ratio (defined by the followingexpression) in the range of 0.1% to 2.5%.(Internal cell ratio) [%]={(real density)−(apparent density)}/(realdensity)×100

What is meant by “real density” in the present invention is density(unit; [g/cm³]) which is fixedly determined from chemical composition(repeating unit of a polymer, minute raw materials such as thecrosslinking agent, graft component used arbitrarily, etc.) of waterabsorbent polyacrylic acid (salt) resin which is sufficiently dried(water content of preferably less than 1 wt %, more preferably less than0.5 wt %, and especially preferably less than 0.1 wt %). Therefore, thereal density of the water absorbent polyacrylic acid (salt) resin issubstantially constant, even though it may vary slightly due to itsneutralization rate, the type of salt of the neutralization (forexample, sodium polyacrylate having a neutralization rate of 75 mol %),or the minute raw materials.

What is meant by “apparent density” in the present invention is density(unit; [g/cm³]) determined in consideration of pores (in other words,internal cells or closed cells) inside the particles of the waterabsorbent polyacrylic acid (salt) resin. For example, water absorbentresin obtained by foaming polymerization or water absorbent resin havingbeen subjected to the granulation step has a space (void; internal cell,closed cell; closed pore) inside, which space is not communicated withits outside, as illustrated in FIG. 2. Thus, when the density of thewater absorbent resin is measured by dry density measurement, theapparent density is obtained from the volume including the closed pore(closed cells) because introduced gas cannot enter the closed pore.Regarding the apparent density of water absorbent resin, Non-PatentLiterature 1, pages 197 to 199, discloses that water absorbent resinhaving been subjected to 40 to 60 mesh-cut is measured by wetmeasurement in which volume of the water absorbent resin is measured byuse of methanol. The apparent density of the present invention ischaracterized in being measured by the dry measurement for all particlediameters. It was found that the internal cell ratio defined by suchapparent density is important for improvement of the properties of thewater absorbent resin.

The density (real density and apparent density) of the water absorbentresin can be accurately measured by the dry density measurement in whicha certain gas is used. The dry density measurement for solid is based onsuch measurement principle that has been well known in an isovolumetricswelling method in which volume of the solid is measured by use of acertain gas. More specifically, assuming that the volume of cells of asample, V_(cell), and the volume of the cells expanded, V_(exp), areknown, the volume of the sample, V_(samp), can be obtained by measuringpressures (gage pressures) P_(1g) and P_(2g), and the density of thesample can be obtained by dividing the volume of the sample by mass ofthe sample, which is separately measured (see the homepage of ShimazuCorporation, http://www.shimadzu.co.jp/powder/lecture/middle/m04.ht ml).

The real density is fixedly determined from the chemical composition(mainly, the repeating unit of the polymer). Thus, a known value may beused as the real density. If there is no known value for the realdensity of the water absorbent resin because the real density is variedslightly due to the minute raw materials of the water absorbent resin,the real density may be determined by a later-described method. Thewater absorbent resin has substantially no closed cell by beingsubjected to eliminating by which the closed cells in the waterabsorbent resin are broken or converted into open cells. Therefore, thedensity of the water absorbent resin thus subjected to eliminating canbe regarded as the real density. Here, the “open cells” are cellscommunicating with outside, and are not measured into the volume of thewater absorbent resin in the dry density measurement. Thus, the closedcells and the open cells can be easily distinguished from each other bythe dry density measurement.

An unpublished prior application PCT/JP2010/073254 (InternationalApplication Date: Dec. 24, 2010) describes producing water absorbentresin, into which fine cells (closed cells) are introduced, by adding aninterfacial active agent into a monomer aqueous solution, and thenpolymerizing the monomer aqueous solution. The international applicationdescribes employing an internal cell ratio for calculating how much theclosed cells account for in the water absorbent resin, and alsodescribes that the internal cell ratio is preferably in a range of 2.8%to 6.6%. Further, the internal application describes that the range ofthe internal cell ratio makes it possible to improve a water absorbingrate (FSR) and permeability potential (SFC) of the water absorbent resinwhich are inversely proportional to each other.

The international application, however, discloses neither the productionmethod of the present invention (the gel grinding, the drying, and thesurface treatment under the specific conditions) nor the water absorbentresin powder of the present invention to be produced by, for example,the production method of the present invention. That is, conventionalknowledge such as the international application does not disclose thewater absorbent resin powder having (i) the internal cell ratio in therange of 0.1% to 2.5%, (ii) the water absorbing rate (FSR) of not lessthan 0.30 [g/g/s], and (iii) the absorption against pressure of not lessthan 20 [g/g].

The water absorbent resin powder of the present invention thus has (i)the internal cell ratio in the range of 0.1% to 2.5%, (ii) theabsorption against pressure (AAP) of not less than 20 [g/g], and (iii)the water absorbing rate (FSR) of not less than 0.30 [g/g/s]. The waterabsorbent resin powder having such properties yields a new effect ofgreatly improving the permeability potential (SFC) while keeping thewater absorbing rate (FSR).

The water absorbent resin powder of the present invention has the rangeof the internal cell ratio smaller than that defined in theinternational application (earlier application). As the internal cellratio approaches 0%, a difference between the real density and theapparent density becomes small. This means that the water absorbentresin includes less closed cells accordingly. In other words, thesurface area of the water absorbent resin is to be reduced. It istherefore supposed that the water absorbing rate (FSR) that depends onthe surface area is decreased. However, the water absorbing rate (FSR)of the water absorbent resin powder of the present invention is notdecreased on the contrary.

As the reason why the water absorbing rate (FSR) of the water absorbentresin powder of the present invention is not decreased, it is consideredthat the surface area is increased by a factor other than the closedcells. Examples of the factor encompass an uneven surface of the waterabsorbent resin, and holes in a part of the water absorbent resin. Thewater absorbent resin having such a surface, and further having a highAAP, that is, a sufficient gel strength under pressure makes it possibleto increase a space in the water absorbent resin powder after beingswollen, thereby improving the permeability potential (SFC) underpressure.

The internal cell ratio of the water absorbent resin of the presentinvention is in a range of 0.1% to 2.5%, preferably in a range of 0.2%to 2.0%, more preferably in a range of 0.3% to 1.7%, and furtherpreferably in a range of 0.5% to 1.5%. Controlling the internal cellratio in the above range makes it possible to obtain the water absorbentresin having the water absorbing rate and the permeability potentialwhich are defined in the present invention. The internal cell ratio canbe controlled by the gel grinding energy, increase in water solublecontent, etc. in the production method of the present invention.Alternatively, the internal cell ratio may be controlled by means(combination use) of foaming polymerization, foaming during drying, orthe like.

(Other Properties)

The water absorbent resin obtainable by the production method of thepresent invention is preferably configured to satisfy the followingproperties. When the water absorbent resin whose main component is waterabsorbent polyacrylic acid (salt) resin is to be used in sanitary goods,especially, disposable diapers, it is controlled by the polymerizationmethod or the surface crosslinking method so as to satisfy preferably(i) at least one of (3-1) to (3-8), more preferably (ii) AAP and atleast one other of (3-1) to (3-8), or especially preferably (iii) AAPand at least two others of (3-1) to (3-8). If the following propertieswere not satisfied, the water absorbent resin would not be able tosufficiently perform in a high-concentration disposable diaper havingwater absorbent resin concentration of not less than 40 wt %.

(3-1) AAP (Absorption Against Pressure)

In order to prevent leakage in disposable diapers, the water absorbentresin obtainable by the present invention under load of 4.8 kPa hasabsorption against pressure (AAP) of preferably 20 [g/g] or more, morepreferably 22 [g/g] or more, and further preferably 24 [g/g] or more,for example by the polymerization. An upper limit of the AAP is notparticularly limited. However, considering a balance with the otherproperties, the upper limit is preferably 35 [g/g] or less, morepreferably 30 [g/g] or less, and further preferably 28 [g/g] or less.Note that the AAP can be improved by the surface crosslinking after theparticle diameter control, and it is possible to obtain the novel waterabsorbent resin of the present invention, and improve the permeabilitypotential (SFC) while keeping the water absorbing rate (FSR), byperforming the surface crosslinking so that the AAP becomes in the aboverange.

(3-2) SFC (Saline Flow Conductivity)

In order to prevent the leakage in disposable diapers, saline flowconductivity (SFC) of the water absorbent resin obtainable by thepresent invention can be improved by the production method of thepresent invention, particularly by the surface crosslinking after thegel grinding, preferably after the particle diameter control of thepresent invention. The SFC (permeability potential of a liquid againstpressure) for a 0.69 wt % sodium chloride aqueous solution is controlledto be preferably 1 [×10⁻⁷·cm³·sec·g⁻¹] or more, more preferably 20[×10⁻⁷·cm³·sec·g⁻¹] or more, further preferably 50 [×10⁻⁷·cm³·sec·g⁻¹]or more, especially preferably 70 [×10⁻⁷·cm³·sec·g⁻¹] or more, and mostpreferably 100 [×10⁻⁷·cm³·sec·g⁻¹] or more, for example by the surfacecrosslinking so that the AAP becomes in the above range. The SFC is awell-known measuring method, and can be defined by, for example, themethod described in U.S. Pat. No. 5,562,646. The present invention issuitably applicable to the production of the water absorbent resin witha high permeability potential, because the present invention isremarkably effective to attain permeability potential improvement,especially SFC improvement, especially to attain SFC within the aboverange, or specifically SFC of 20 [×10⁻⁷·cm³·sec·g⁻¹] or more.

(3-3) CRC (Absorbency Without Pressure)

CRC (absorbency without pressure) of the water absorbent resinobtainable by the present invention is preferably 10 [g/g] or more, morepreferably 20 [g/g] or more, further preferably 25 [g/g] or more, andespecially preferably 30 [g/g] or more. An upper limit of the CRC is notlimited. However, considering the balance with the other properties, theupper limit is preferably 50 [g/g] or less, more preferably 45 [g/g] orless, and further preferably 40 [g/g] or less. The CRC can beappropriately controlled by crosslinking agent content during thepolymerization, and the surface crosslinking (secondary crosslinking)after the polymerization.

(3-4) Ext (Water Soluble Content)

In order to prevent disposable diapers from growing sticky etc. in usedue to leakage of liquid, Ext (water soluble content) of the waterabsorbent resin obtainable by the present invention is preferably 35 wt% or less, more preferably 25 wt % or less, further preferably 15 wt %or less, and especially preferably 10 wt % or less. The Ext can beappropriately controlled by the crosslinking agent content during thepolymerization, and increase in water soluble content due to the gelgrinding after the polymerization.

(3-5) Residual Monomers

Residual monomers of the water absorbent resin obtainable by the presentinvention are controlled to be normally 500 ppm or less, preferably in arange of 0 to 400 ppm, more preferably in a range of 0 to 300 ppm, andespecially preferably in a range of 0 to 200 ppm, for example by thepolymerization, in terms of safety. The residual monomers can beappropriately controlled by a polymerization initiator during thepolymerization, drying conditions after the polymerization, etc.

(3-6) FSR (Water Absorbing Rate)

In order to prevent the leakage in disposable diapers, the waterabsorbent resin obtainable by the present invention has a waterabsorbing rate (FSR) of normally 0.2 [g/g/s] or more, preferably 0.25[g/g/s] or more, more preferably 0.30 [g/g/s] or more, furtherpreferably 0.35 [g/g/s] or more, especially preferably 0.40 [g/g/s] ormore, and most preferably 0.45 [g/g/s] or more, for example by thepolymerization. An upper limit of the FSR is 1.00 [g/g/s] or less. Themeasuring method of the FSR is defined by PCT International PublicationNo. 2009/016055. The FSR can be controlled by the first to fourthproducing methods of the present invention, and the particle diametercontrol after drying.

The present invention is suitably applicable to the production of thewater absorbent resin with a high water absorbing rate, because thepresent invention is remarkably effective to attain water absorbing rateimprovement, especially FSR improvement, especially to attain FSR withinthe above range, specifically FSR of 0.30 [g/g/s] or more.

(3-7) Amount of Fine Powder to Increase Before and After Damage

An amount of fine powder to increase before and after damage in thewater absorbent resin obtainable by the present invention, which amountis defined by the measurement method of Examples (an amount of particlespassing through a sieve of 150 μm to increase) is preferably in a rangeof 0 to 3 wt %, and more preferably in a range of 0 to 1.5 wt %. Thewater absorbent resin including the fine powder in such a range does nothave a problem of decrease in the properties in being actually used suchas being used in disposable diapers. The amount of fine powder toincrease is lowered by the first to fourth producing methods (gelgrinding) of the present invention.

(3-8) Bulk Specific Gravity

Bulk specific gravity (defined by ERT460.2-02) of the water absorbentresin obtainable by the present invention is preferably in a range of0.50 [g/cm³] to 0.80 [g/cm³], and further preferably in a range of 0.60[g/cm³] to 0.70 [g/cm³]. Water absorbent resin having no bulk specificgravity in the range may deteriorate its properties or may be powdered.The bulk specific gravity can be lowered by the first to fourthproducing methods (gel grinding) of the present invention.

(3-9) Surface Crosslinking

In order to attain the object of the present invention, the waterabsorbent resin is preferably subjected to surface crosslinking,particularly crosslinked by combination use of the ionic bonding surfacecrosslinking agent (such as polyvalent metal) and the covalent bondingsurface crosslinking agent. Note that in the present invention, thewater absorbent resin that is subjected to the surface crosslinking maybe referred to as water absorbent resin powder.

[4] Application of Water Absorbent Polyacrylic Acid (Salt) Resin Powder

The water absorbent resin powder obtainable by the production method ofthe present invention is not limited to particular applications, but ispreferably applicable to absorbing products such as disposable diaper,sanitary napkins, and incontinence pad. The water absorbent resin powdershows an excellent property in a case where it is used in a highconcentration diaper (a disposable diaper in which a lot of waterabsorbent resin is used) having problems such as odor derived from amaterial, and coloring, particularly in a case where it is used in anupper layer part of an absorber of the high concentration disposablediaper.

The absorbing products, which may arbitrary contain other absorbingmaterial(s) (such as pulp or fibers), have water absorbent resin content(core concentration) preferably in a range of 30 wt % to 100 wt %, morepreferably in a range of 40 wt % to 100 wt %, further preferably in arange of 50 wt % to 100 wt %, further more preferably in a range of 60wt % to 100 wt %, especially preferably in a range of 70 wt % to 100 wt%, and most preferably in a range of 75 wt % to 95 wt %. For Example, ina case where the water absorbent resin powder obtainable by theproduction method of the present invention is used with the above coreconcentration particularly in the upper part of the absorber, liquid isefficiently distributed in the absorber, and an amount of the liquid tobe absorbed by the entire absorbing product is improved because theabsorber has an excellent diffusivity of absorbed liquid such as urinethanks to a high permeability potential of the absorber. It is furtherpossible to provide the absorbing product in which the absorber keepswhite color giving an impression of cleanness.

That is, the present application includes the following invention.

[1] A method for producing water absorbent polyacrylic acid (salt) resinpowder, including the steps of: (i) polymerizing an acrylic acid (salt)monomer aqueous solution; (ii) during or after the step of (i),performing gel grinding of a hydrogel crosslinked polymer obtained bythe polymerization, wherein the hydrogel crosslinked polymer has resinsolid content of 10 wt % to 80 wt %; (iii) drying a particulate hydrogelcrosslinked polymer obtained by the gel grinding, wherein the drying isperformed at 150° C. to 250° C. by use of a drier; and (iv) carrying outa surface treatment to the particulate hydrogel crosslinked polymer thusdried, the step of (ii) being carried out such that at least one of (1)to (4) is met, where (1) the gel grinding is carried out with gelgrinding energy (GGE) of 18 [J/g] to 60 [J/g]; (2) the gel grinding iscarried out with gel grinding energy (2) (GGE (2)) of 9 [J/g] to 40[J/g]; (3) weight average molecular weight of water soluble content ofthe hydrogel crosslinked polymer is increased by 10,000 [Da] to 500,000[Da]; and (4) the particulate hydrogel crosslinked polymer obtained bythe step of (ii) has a weight average particle diameter (D50) of 350 μmto 2000 μm, and logarithmic standard deviation (σζ) of particle sizedistribution of 0.2 to 1.0.

In a case where the hydrogel crosslinked polymer is subjected to the gelgrinding so that (4) is met, the particulate hydrogel crosslinkedpolymer is dried by a through-flow belt drier, the particulate hydrogelcrosslinked polymer to be supplied into the through-flow belt drier hasresin solid content of 10 wt % to 80 wt %, and the through-flow beltdrier sends hot air of 150° C. to 250° C. at a velocity of 0.8 [m/s] to2.5 [m/s] in a direction vertical (up-and-down direction) to theparticulate hydrogel crosslinked polymer.

[2] A method for producing water absorbent polyacrylic acid (salt) resinpowder, including the steps of: (i) polymerizing an acrylic acid (salt)monomer aqueous solution; (ii) during or after the step of (i),performing gel grinding of a hydrogel crosslinked polymer obtained bythe polymerization, wherein the hydrogel crosslinked polymer has resinsolid content of 10 wt % to 80 wt %, and the gel grinding is carried outwith gel grinding energy (GGE) of 18 [J/g] to 60 [J/g]; (iii) drying at150° C. to 250° C. a particulate hydrogel crosslinked polymer obtainedby the gel grinding; and (iv) carrying out a surface treatment to theparticulate hydrogel crosslinked polymer thus dried.

[3] A method for producing water absorbent polyacrylic acid (salt) resinpowder, including the steps of: (i) polymerizing an acrylic acid (salt)monomer aqueous solution; (ii) during or after the step of (i),performing gel grinding of a hydrogel crosslinked polymer obtained bythe polymerization, wherein the hydrogel crosslinked polymer has resinsolid content of 10 wt % to 80 wt %, and the gel grinding is carried outwith gel grinding energy (2) (GGE (2)) of 9 [J/g] to 40 [J/g]; (iii)drying at 150° C. to 250° C. a particulate hydrogel crosslinked polymerobtained by the gel grinding; and (iv) carrying out a surface treatmentto the particulate hydrogel crosslinked polymer thus dried.

[4] A method for producing water absorbent polyacrylic acid (salt) resinpowder, including the steps of: (i) polymerizing an acrylic acid (salt)monomer aqueous solution; (ii) during or after the step of (i),performing gel grinding of a hydrogel crosslinked polymer obtained bythe polymerization, wherein the hydrogel crosslinked polymer has resinsolid content of 10 wt % to 80 wt %, so that weight average molecularweight of water soluble content of the hydrogel crosslinked polymer isincreased by 10,000 [Da] to 500,000 [Da]; (iii) drying at 150° C. to250° C. a particulate hydrogel crosslinked polymer obtained by the gelgrinding; and (iv) carrying out a surface treatment to the particulatehydrogel crosslinked polymer thus dried.

[5] The method as set forth in any one of [2] to [4], wherein theparticulate hydrogel crosslinked polymer obtained by the step of (ii)has resin solid content of 10 wt % to 80 wt %.

[6] The method as set forth in any one of [2] to [5], wherein a drierused in the step of (iii) is a through-flow belt drier configured tosend hot air at a velocity of 0.8 [m/s] to 2.5 [m/s] in a directionvertical to the particulate hydrogel crosslinked polymer.

[7] A method for producing water absorbent polyacrylic acid (salt) resinpowder, including the steps of: (i) polymerizing an acrylic acid (salt)monomer aqueous solution; (ii) during or after the step of (i),performing gel grinding of a hydrogel crosslinked polymer obtained bythe polymerization so as to obtain a particulate hydrogel crosslinkedpolymer having a weight average particle diameter (D50) of 350 μm to2000 μm and logarithmic standard deviation (σζ) of particle sizedistribution of 0.2 to 1.0; (iii) drying the particulate hydrogelcrosslinked polymer by hot air of 150° C. to 250° C. at a velocity of0.8 [m/s] to 2.5 [m/s] in a direction vertical to the particulatehydrogel crosslinked polymer by use of a through-flow belt drier, theparticulate hydrogel crosslinked polymer to be supplied into thethrough-flow belt drier having resin solid content of 10 wt % to 80 wt%; and (iv) carrying out a surface treatment to the particulate hydrogelcrosslinked polymer thus dried.

[8] The method as set forth in any one of [4] to [7], wherein the stepof (ii) is such that the hydrogel crosslinked polymer is subjected togel grinding with gel grinding energy (GGE) of 18 [J/g] to 60 [J/g].

[9] The method as set forth in any one of [1] to [8], wherein the stepof (i) performs kneader polymerization or belt polymerization.

[10] The method as set forth in any one of [1] to [9], wherein the stepof (ii) is carried out with respect to: (a) a hydrogel crosslinkedpolymer having gel CRC of 10 [g/g] to 35 [g/g]; (b) a hydrogelcrosslinked polymer having gel Ext of 0.1 wt % to 10 wt %; or (c) ahydrogel crosslinked polymer having gel CRC of 10 [g/g] to 35 [g/g] andgel Ext of 0.1 wt % to 10 wt %.

[11] The method as set forth in any one of [1] to [10], wherein the stepof (ii) is carried out with respect to a hydrogel crosslinked polymerhaving resin solid content of 40 wt % to 80 wt %.

[12] The method as set forth in any one of [1] to [11], wherein the stepof (ii) is carried out with respect to: (d) a hydrogel crosslinkedpolymer having a monomer polymerization ratio of not less than 90 mol %;(e) a hydrogel crosslinked polymer having a neutralization ratio of 45mol % to 90 mol %; or (f) a hydrogel crosslinked polymer having amonomer polymerization ratio of not less than 90 mol % and aneutralization ratio of 45 mol % to 90 mol %.

[13] The method as set forth in any one of [1] to [12], wherein the stepof (ii) is carried out with respect to a hydrogel crosslinked polymerhaving a gel temperature of 40° C. to 120° C.

[14] The method as set forth in any one of [1] to [13], wherein the stepof (ii) employs a screw extruder including a casing having an edge wherea porous die is provided.

[15] The method as set forth in any one of [1] to [14], wherein the stepof (ii) is carried out so that gel Ext of the hydrogel crosslinkedpolymer is increased by not more than 5 wt %.

[16] The method as set forth in any one of [1] to [15], wherein in thestep of (ii), 0 to 4 part(s) by weight of water is added to 100 parts byweight of the hydrogel crosslinked polymer.

[17] The method as set forth in any one of [1] to [16], wherein theparticulate hydrogel crosslinked polymer thus obtained in the step of(ii) has at least one property of: (g) gel Ext of 0.1 wt % to 10 wt %;(h) gel CRC of 10 [g/g] to 35 [g/g]; and (i) resin solid content of 10wt % to 80 wt %.

[18] The method as set forth in any one of [1] to [17], wherein theparticulate hydrogel crosslinked polymer to be supplied into athrough-flow belt drier in the step of (iii) has a temperature of 60° C.to 110° C.

[19] The method as set forth in any one of [1] to [18], furtherincluding a classifying step, the classifying step providing classifiedwater absorbent resin particles having a weight average particlediameter (D50) of 250 μm to 500 μm and logarithmic standard deviation(σζ) of particle size distribution of 0.25 to 0.50.

[20] The method as set forth in any one of [1] to [19], wherein thesurface treatment includes a surface crosslinking step of performingsurface crosslinking to the particulate hydrogel crosslinked polymerthus dried, and a step of adding, to the particulate hydrogelcrosslinked polymer thus dried, at least one of (i) polyvalent metalsalt, (ii) a cationic polymer and (iii) inorganic fine particles, thestep of adding being carried out concurrently or non-concurrently withthe surface crosslinking step.

[21] Water absorbent polyacrylic acid (salt) resin having: 95 wt % ormore of particles whose diameter is not less than 150 μm and less than850 μm; logarithmic standard deviation (σζ) of particle sizedistribution of 0.25 to 0.50; absorption against pressure (AAP) of 20[g/g] or more; a water absorbing rate (FSR) of 0.30 [g/g/s] or more; andan internal cell ratio of 0.1% to 2.5%, which is calculated by thefollowing expression.(Internal cell ratio) [%]={(real density)−(apparent density)}/(realdensity)×100

[22] The water absorbent polyacrylic acid (salt) resin as set forth in[21], further containing at least one of (i) polyvalent metal salt, (ii)a cationic polymer and (iii) inorganic fine particles.

EXAMPLES

The description below deals with the present invention with reference toExamples. The present invention is, however, not construed limitedly tothe Examples. The physical properties mentioned in the claims of thepresent invention and in the Examples were determined under theconditions of room temperature (20 to 25° C.) and a humidity of 50 RH %by an EDANA method and the measurement methods below unless otherwisestated. The electric devices mentioned in the Examples and ComparativeExamples were operated at 200 V or 100 V with use of a 60-Hz powersupply. The description below may, for convenience, use the letter “L”to mean “liter” and the sign “wt %” to mean “percent by weight”.

(a) CRC and Gel CRC

The Examples measured CRC (Centrifuge Retention Capacity) in accordancewith ERT 441.2-02. Specifically, the Examples weighed out 0.200 g of awater absorbent resin, placed the water absorbent resin uniformly in abag (60×60 mm) made of unwoven cloth, heat-sealed the bag, and thenimmersed the bag in 1000 mL of a 0.9 wt % sodium chloride aqueoussolution having a temperature adjusted to 25±3° C. The Examples, 30minutes later, took out the bag, drained the water absorbent resin byusing a centrifugal device (centrifugal device produced by KOKUSANCorporation, model: H-122) under the conditions of 250 G and 3 minutes,and then measured the weight W1 [g] of the bag. The Examples furthercarried out a similar operation involving no water absorbent resin,measured the weight W2 [g] of a corresponding bag for that operation,and calculated CRC (Centrifuge Retention Capacity) by the expression (4)below.

[Mathematical Expression 4]CRC [g/g]={(W1−W2)/(weight of water absorbent resin)}−1   Expression 4

To measure gel CRC, the Examples carried out an operation identical tothe above operation except for use of 0.4 g of hydrogel and a freeswelling time of 24 hours. The Examples further measured the resin solidcontent of the hydrogel through a separate operation to determine theweight of a water absorbent resin in the above 0.4 g of hydrogel, andcalculated gel CRC in correspondence with the expression (5) below. TheExamples made this measurement 5 times for each sample, and employed theaverage value calculated from the five measurements.

[Mathematical Expression 5]Gel CRC [g/g]={(mwi−mb)−msi×(Wn/100)}/{msi×(Wn/100)}   Expression 5

In the above expression,

msi represents the weight [g] of a hydrogel before measurement;

mb represents the weight [g] of Blank (unwoven cloth only) after freeswell and draining;

mwi represents the weight [g] of the hydrogel after free swell anddraining; and

Wn represents the solid content [wt %] of the hydrogel.

(b) Ext and Gel Ext

The Examples measured Ext (water-soluble content) in accordance with ERT470.2-02. Specifically, the Examples placed 1.000 g of a water absorbentresin and 200 ml of a 0.90 wt % sodium chloride aqueous solution in aplastic vessel (capacity: 250 mL) having a lid and, stirred the mixturewith a cylindrical stirrer (length: 3.5 cm, diameter: 6 mm) at 400 rpmfor 16 hours, and extracted water soluble content from the waterabsorbent resin. The Examples filtered the extract with a filter paper(produced by Advantec Toyo Kaisha, Ltd., product name: JIS P 3801, No.2, thickness: 0.26 mm, retaining particle diameter: 5 μm), and used 50.0g of the filtrate as a measurement liquid.

The Examples then titrated the measurement liquid with a 0.1N-NaOHaqueous solution until the measurement liquid reached a pH of 10,further titrated the measurement liquid with a 0.1N-HCl aqueous solutionuntil the measurement liquid reached a pH of 2.7, and then determinedtiters ([NaOH] mL and [HCl] mL) at that stage. The Examples also carriedout a similar operation with respect to only a 0.90 wt % sodium chlorideaqueous solution to determine blank titers ([bNaOH] mL and [bHCl] mL).The Examples calculated Ext (water soluble content) of a water absorbentresin of the present invention by the expression (6) below on the basisof (i) the average molecular weight of the monomer of the waterabsorbent resin and (ii) the titers calculated through the aboveoperation.

[Mathematical Expression 6]Ext [wt %]=0.1×(average molecular weight ofmonomer)×200×100×([HCl]−[bHCl])/1000/1.000/50.0   Expression 6

To calculate gel Ext, the Examples used 5.0 g of hydrogel cut withscissors into a shape having sides each with a length of approximately 1mm to 5 mm, and carried out an operation identical to the aboveoperation except for a stirring time of 24 hours. The Examples furthermeasured the resin solid content of the hydrogel in a separate operationto determine the weight of the water absorbent resin of the above 5.0 gof hydrogel, and calculated gel Ext by the expression (7) below.

[Mathematical Expression 7]Gel Ext [wt %]={(V _(HCl.s) −V _(HCl.b))×C _(HCl) ×Mw×F_(dil)×100}/ms×(Wn/100)×1000   Expression 7

In the above expression,

V_(HCl.s) represents the amount [ml] of HCl necessary to reduce the pHfrom 10 to 2.7 of a filtrate including a dissolved polymer;

V_(HCl.b) represents the amount [ml] of HCl necessary to reduce the pHfrom 10 to 2.7 of Blank (0.9 wt % sodium chloride aqueous solution);

C_(HCl) represents the concentration [mol/1] of an HCl solution;

Mw represents the average molecular weight [g/mol] of a monomer unit inacrylic acid (salt) polymer (for example, Mw is 88.1 [g/mol] in the caseof a neutralization rate of 73 mol %);

F_(dil) represents the dilution of a filtrate including a dissolvedpolymer;

ms represents the weight [g] of a hydrogel before the measurement; and

Wn represents the solid content [wt %] of the hydrogel.

(c) Weight Average Molecular Weight of Water Soluble Content

The weight average molecular weight of the water soluble content isexpressed as a value obtained by a GPC measurement of the weight averagemolecular weight of a polymer dissolved through the above operation ofmeasuring Ext and gel Ext. The following describes that GPC measurement.

The Examples used, for GPC, TDA 302 (registered trademark) produced byViscotech Co., Ltd. This device includes a size exclusionchromatography, a refractive index detector, a light scatteringdetector, and a capillary viscometer. The Examples used the measurementdevices and measurement conditions listed below.

Pump-autosampler: GPC max produced by Viscotech Co., Ltd.

Guard column: SHODEX GF-7B

Column: two TOSOH GMPWXLs connected in series

Detector: TDA 302 produced by Viscotech Co., Ltd. (temperature insidethe system was kept at 30° C.)

Solvent: aqueous solution including 60 mM of sodium dihydrogen phosphatedihydrate and 20 mM of disodium hydrogenphosphate dodecahydrate

Flow rate: 0.5 ml/min

Injection amount: 100 μl

The Examples used, for device calibration, polyoxyethylene glycol(weight average molecular weight (Mw): 22396, differential refractiveindex (dn/dc)=0.132, solvent refractive index of 1.33) as a referencesample.

In the case where the measurement target substance was a water absorbentresin prepared by polymerizing a monomer containing 99 or more mol %acrylic acid (salt), the Examples set the differential refractive index(dn/dc) of a polymer as an analysis target to 0.12 to carry outmeasurements. In the case where the measurement target substance was acopolymerized water absorbent resin having a 1 or more mol % content ofa monomer other than acrylic acid (salt), the Examples measured adifferential refractive index (dn/dc) unique to that polymer andobserved when the polymer was in a solvent, and carried out measurementswith use of that numerical value. The Examples collected and analyzeddata on the refractive index, light scatter intensity, and viscositywith use of the Viscotek OmniSEC 3.1 (registered trademark) software.The Examples calculated the weight average molecular weight (Mw) on thebasis of data obtained from the refractive index and light scatterintensity.

(d) Weight Average Particle Diameter (D50) and Logarithmic StandardDeviation (σζ) of Particle Size Distribution

The Examples measured the weight average particle diameter (D50) and thelogarithmic standard deviation (σζ) of a particle size distribution fora water absorbent resin in accordance with a measurement methoddescribed in European Patent No. 0349240. The Examples measured theweight average particle diameter (D50) and the logarithmic standarddeviation (σζ) of a particle size distribution for a hydrogel by amethod below.

Specifically, the Examples added, to 500 g of a 20 wt % sodium chlorideaqueous solution containing 0.08 wt % of Emal 20C (hereinafter referredto as “Emal aqueous solution”) (surface active agent, produced by KaoCorporation), 20 g of hydrogel (solid content: a wt %) with atemperature of 20° C. to 25° C. to prepare a dispersion solution, andstirred the dispersion solution with a stirrer chip (length: 50 mm,diameter: 7 mm) at 300 rpm for 60 minutes in a columnar polypropylenevessel (height: 21 cm, diameter: 8 cm) having a capacity ofapproximately 1.14 L.

The Examples, after finishing the stirring, placed the above dispersionsolution on a central portion of JIS standard sieves (diameter: 21 cm,mesh sizes of the sieves: 8 mm/4 mm/2 mm/1 mm/0.60 mm/0.30 mm/0.15mm/0.075 mm) disposed on a turntable. The Examples washed all thehydrogel with use of 100 g of the Emal aqueous solution so that thehydrogel would appear on the sieves, and then uniformly poured 6000 g ofthe Emal aqueous solution from above at the height of 30 cm with use ofa shower (with 72 holes, fluid volume: 6.0 [L/min]) while manuallyrotating the sieves at 20 rpm so that the water-pouring range (50 cm²)would cover the entire sieves, and thus classified the hydrogel. TheExamples drained the classified hydrogel on a first-stage sieve forapproximately 2 minutes, and weighed the classified hydrogel. TheExamples carried out a similar operation with respect to sieves onsecond and subsequent sieves for classification, and weighed thehydrogel remaining on each sieve after draining it.

The Examples calculated a ratio (wt %) of the hydrogel remaining on eachsieve on the basis of its weight by the expression (8) below. TheExamples used, after the draining, sieves having the mesh sizes definedby the expression (9) below, and plotted the particle size distributionof the hydrogel on a logarithmic probability paper. The Examples used,as the weight average particle diameter (D50) of the hydrogel, aparticle diameter corresponding to 50 wt % which is calculated by addingup the ratios of the hydrogels remaining on the respective sieves on theplot. The Examples further determined, from the plot, particle diametersrespectively corresponding to 84.1 wt % (designated by X1) and 15.9 wt %(designated by X2) to determine the logarithmic standard deviation (σζ)by the expression (10) below. A smaller value of σζ indicates a narrowerparticle size distribution.

[Mathematical Expression 8]x [%]=(w/W)×100   Expression 8[Mathematical Expression 9]R(α) [mm]=(20/w)^(1/3) ×r   Expression 9

In the above expressions,

X represents the weight [%] of a hydrogel remaining on each sieve afterdraining and classification;

w represents the weight [g] of an individual hydrogel remaining on eachsieve after draining and classification;

W represents the total weight [g] of hydrogel remaining on the sievesafter draining and classification;

R(α) represents the mesh size [mm] of a sieve which mesh size is asconverted to correspond to a case of a hydrogel with a solid content ofa wt %; and

r represents the mesh size [mm] of a sieve through which is classified ahydrogel that has swollen in a 20 wt % sodium chloride aqueous solution.

[Mathematical Expression 10]σζ=0.5×ln(X2/X1)   Expression 10

X1 represents a particle diameter in the case where R=84.1%, and X2represents a particle diameter in the case where R=15.9%.

(e) Apparent Density

The Examples removed moisture from the water absorbent resin, andmeasured (dry measurement of the volume of a water absorbent resinhaving a predetermined weight), with use of a dry densimeter, anapparent density that takes into consideration cells (internal cells)present inside the resin.

Specifically, the Examples weighed out 6.0 g of the water absorbentresin, placed it in an aluminum cup having a bottom surface with adiameter of approximately 5 cm, and then dried the water absorbent resinin a no-air flow drier at 180° C. The Examples left the water absorbentresin for 3 hours or longer until its moisture content was not greaterthan 1 wt %, and thus sufficiently dried the water absorbent resin. TheExamples, after the drying, measured the apparent density (unit:[g/cm³]) of 5.00 g of the water absorbent resin with use of a dryautomatic densimeter (AccuPycII 340TC-10CC, produced by ShimazuCorporation, carrier gas: helium). The Examples repeated thatmeasurement until the measured values were the same continuously for 5or more times.

(f) Real Density

Internal cells (closed cells) present inside a water absorbent resineach normally have a diameter of 1 μm to 300 μm. Such a water absorbentresin is ground preferentially at portions close to closed cells. In thecase where a water absorbent resin has been ground to have a particlediameter of 45 μm or less, the resulting water absorbent resin hasalmost no closed cells (see FIG. 3). The present invention thusevaluated, as a real density, a dry density of a water absorbent resinthat had been ground to have a particle diameter of 45 μm or less.

Specifically, the Examples placed 15.0 g of the water absorbent resinand 400 g of columnar ceramic balls (each with a diameter of 13 mm and alength of 13 mm) in a ball mill pot (produced by Teraoka Corporation,type No. 90/internal size, diameter: 80 mm, height: 75 mm; externalsize, diameter: 90 mm, height: 110 mm), and operated the ball mill potat 60 Hz for 2 hours to prepare a water absorbent resin that would passthrough a JIS standard sieve having a mesh size of 45 μm (that is, awater absorbent resin having a particle diameter of 45 μm or less). TheExamples dried 6.0 g of that water absorbent resin with a particlediameter of 45 μm or less in a manner similar to the manner describedunder [Apparent Density] above, that is, at 180° C. for 3 hours orlonger, and then measured the dry density of the water absorbent resin.The value thus measured was designated as the “real density” of thepresent invention.

(g) Internal Cell Rate

The Examples calculated the internal cell rate of the water absorbentresin by the expression (11) below on the basis of (i) the apparentdensity (designated by ρ1 [g/cm³]) measured by the method describedunder [Apparent Density] above and (ii) the real density (designated byρ2 [g/cm³]) measured by the method described under [Real Density] above.

[Mathematical Expression 11]Internal cell rate [%]=(ρ2−ρ1)/ρ2×100   Expression 11

Production Example 1

The present Production Example prepared, as a device for producing waterabsorbent polyacrylic acid (salt) resin powder, a continuous productiondevice for carrying out a polymerization step, a gel grinding step, adrying step, a grinding step, a classifying step, a surface crosslinkingstep, a cooling step, a granulating step, and a transporting step forlinking the above individual steps. The continuous production device hada production capacity of approximately 3500 [kg/hr]. The above steps mayeach include a single system or two or more systems. In the case wherethe above steps each include two or more systems, the productioncapacity is shown as the total of the respective production amounts ofthe individual systems. The present Production Example used thiscontinuous production device to continuously produce water absorbentpolyacrylic acid (salt) resin powder.

The present Production Example prepared a monomer aqueous solution (1)containing (i) 193.3 parts by weight of acrylic acid, (ii) 64.4 parts byweight of a 48 wt % sodium hydroxide aqueous solution, (iii) 1.26 partsby weight of polyethyleneglycol diacrylate (average number n=9), (iv) 52parts by weight of a 0.1 wt % pentasodium ethylenediaminetetra(methylene phosphonate) aqueous solution, and (v) 134 parts byweight of deionized water.

The present Production Example next continuously fed, with use of aconstant rate pump, the above monomer aqueous solution (1) with atemperature adjusted to 40° C., and then further continuously fed 97.1parts by weight of a 48 wt % sodium hydroxide aqueous solution for linemixing. The temperature of the monomer aqueous solution (1) was at thisstage raised to 85° C. due to heat of neutralization.

Further, the present Production Example next continuously fed 8.05 partsby weight of a 4 wt % sodium persulfate aqueous solution for linemixing, and then continuously fed the resulting mixture to a continuouspolymerization device, equipped with a planar polymerization beltprovided with a dam at each end, so that the fed mixture would have athickness of approximately 7.5 mm. The continuous polymerization devicecarried out polymerization continuously (polymerization period: 3minutes) to prepare a hydrogel (1) in the shape of a belt. Thebelt-shaped hydrogel (1) had CRC of 28.0 [g/g], resin solid content of53.0 wt %, water soluble content of 4.0 wt %, and weight averagemolecular weight of the water soluble content of 218,377 [Da].

Comparative Example 1

The present Comparative Example continuously produced water absorbentpolyacrylic acid (salt) resin powder subsequently to Production Example1.

Specifically, the present Comparative Example continuously cut, with acutting length of approximately 300 mm, the belt-shaped hydrogel (1),prepared in Production Example 1, at equal intervals along the widthdirection with respect to the direction in which the polymerization beltmoved.

The present Comparative Example fed the hydrogel (1) with the cuttinglength of approximately 300 mm to a screw extruder to grind the gel. Thescrew extruder was a meat chopper that (i) included, at an end, a porousdie having a diameter of 340 mm, a porous diameter of 22 mm, 105 pores,a hole area rate of 52%, and a thickness of 20 mm and that (ii) had ascrew shaft with a diameter of 152 mm. The present Comparative Example,in the state where the meat chopper was set so that the number ofrevolutions of the screw shaft was 96 rpm, fed 132800 [g/min] of thehydrogel (1) simultaneously with 855.8 [g/min] of 70° C. hot water and3333 [g/min] of water vapor. The present Comparative Example, at thisstage, had gel grinding energy (GGE) of 17.9 [J/g] and gel grindingenergy (2) (GGE (2)) of 8.7 [J/g]. The meat chopper had a current valueof 89.6 A during the gel grinding. The hydrogel (1) had a temperature of90° C. before the gel grinding, whereas after the gel grinding, acomparative ground gel, that is, a comparative particulate hydrogel (1),had a temperature raised to 110° C.

The comparative particulate hydrogel (1) prepared through the above gelgrinding step had CRC of 28.2 [g/g], resin solid content of 49.4 wt %,water soluble content of 4.3 wt %, weight average molecular weight ofthe water soluble content of 225,674 [Da], a weight average particlediameter (D50) of 1041 μm, and logarithmic standard deviation (σζ) ofparticle size distribution of 1.74. Table 1 shows the conditions appliedto the gel grinding step. Table 2 shows physical properties of thecomparative particulate hydrogel (1).

The present Comparative Example next distributed (at this stage, thecomparative particulate hydrogel (1) had a temperature of 80° C.) thecomparative particulate hydrogel (1) on a through-flow belt within 1minute from the end of the gel grinding to dry the comparativeparticulate hydrogel (1) at 185° C. for 30 minutes. This prepared 246parts by weight of a comparative dried polymer (1) (that is, the totalamount of output during the drying step). The through-flow belt had amoving rate of 1 [m/min]. The hot air had an average wind velocity of1.0 [m/s] with respect to the direction perpendicular to the directionin which the through-flow belt moved. The present Comparative Examplemeasured the wind velocity of the hot air with use ofconstant-temperature thermal anemometer Anemomaster 6162 produced byKANOMAX JAPAN incorporated.

The present Comparative Example next continuously fed all thecomparative dried polymer (1), prepared through the drying step andhaving a temperature of approximately 60° C., to a three-stage roll millto grind it (that is, a grinding step), and then classified the groundcomparative dried polymer (1) with use of JIS standard sieves havingrespective mesh sizes of 710 μm and 175 μm. This prepared comparativewater absorbent resin particles (1) each having an irregularly brokenshape. The comparative water absorbent resin particles (1) had a weightaverage particle diameter (D50) of 350 μm, logarithmic standarddeviation (σζ) of particle size distribution of 0.33, CRC of 31.6 [g/g],and water soluble content of 6.8 wt %, and included 150 μm passingparticles (that is, the proportion of particles that would pass througha sieve having a mesh size of 150 μm) at 0.6 wt %.

The present Comparative Example next (i) uniformly mixed, with 100 partsby weight of the comparative water absorbent resin particles (1), a(covalent bonding) surface crosslinking agent solution containing 0.3parts by weight of 1,4-butandiol, 0.6 parts by weight of propyleneglycol, and 3.0 parts by weight of deionized water, and (ii)heat-treated the resulting mixture at 208° C. for approximately 40minutes so that the resulting comparative water absorbent resin powder(1) would have CRC ranging from 26.6 [g/g] to 27.4 [g/g]. The presentComparative Example then cooled the resulting comparative waterabsorbent resin powder (1) and uniformly mixed with it a (ionic bonding)surface crosslinking agent solution containing 1.17 parts by weight of a27.5 wt % aluminum sulfate aqueous solution (8 wt % based on aluminumoxide), 0.196 parts by weight of a 60 wt % sodium lactate aqueoussolution, and 0.029 parts by weight of propylene glycol.

The present Comparative Example next crushed the resulting mixture (thatis, the granulating step) until it would pass through a JIS standardsieve having a mesh size of 710 μm. This prepared comparative waterabsorbent resin powder (1). Table 3 shows physical properties of thecomparative water absorbent resin powder (1).

Example 1

The present Example carried out, with respect to the belt-shapedhydrogel (1) prepared in Production Example 1, an operation identical tothat of Comparative Example 1 except that the gel was ground under theconditions that (i) the cutting length was 200 mm, (ii) no hot water orwater vapor was fed, and (iii) the number of revolutions of the screwshaft of the meat chopper for the gel grinding was changed to 115 rpm.This operation prepared (i) ground gel, that is, particulate hydrogel(1), (ii) water absorbent resin particles (1), and (iii) water absorbentresin powder (1). Example 1 had gel grinding energy (GGE) of 27.8 [J/g]and gel grinding energy (2) (GGE (2)) of 15.5 [J/g]. The meat chopperhad a current value of 104.7 A during the gel grinding. The hydrogel (1)had a temperature of 90° C. before the gel grinding, whereas after thegel grinding, the particulate hydrogel (1) had a temperature lowered to85° C. The particulate hydrogel (1) had a temperature of 75° C. when itwas introduced to a drier.

The particulate hydrogel (1) prepared as above had CRC of 28.3 [g/g],resin solid content of 50.8 wt %, water soluble content of 4.4 wt %,weight average molecular weight of the water soluble content of 253,596[Da], a weight average particle diameter (D50) of 750 μm, andlogarithmic standard deviation (σζ) of particle size distribution of0.79. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the particulate hydrogel (1).

The water absorbent resin particles (1) had a weight average particlediameter (D50) of 340 μm, logarithmic standard deviation (σζ) ofparticle size distribution of 0.32, CRC of 32.0 [g/g], and water solublecontent of 6.9 wt %, and included 150 μm passing particles (that is, theproportion of particles that would pass through a sieve having a meshsize of 150 μm) at 0.7 wt %. Table 3 shows physical properties of thewater absorbent resin powder (1).

Example 2

The present Example carried out, with respect to the belt-shapedhydrogel (1) prepared in Production Example 1, an operation identical tothat of Comparative Example 1 except that the gel was ground under theconditions that (i) the cutting length was 200 mm, (ii) no hot water orwater vapor was fed, and (iii) the number of revolutions of the screwshaft of the meat chopper was changed to 134 rpm. This operationprepared (i) particulate hydrogel (2), (ii) water absorbent resinparticles (2), and (iii) water absorbent resin powder (2). Example 2 hadgel grinding energy (GGE) of 28.2 [J/g] and gel grinding energy (2) (GGE(2)) of 15.8 [J/g]. The meat chopper had a current value of 105.6 Aduring the gel grinding. The hydrogel (2) had a temperature of 90° C.before the gel grinding, whereas after the gel grinding, the particulatehydrogel (2) had a temperature lowered to 86° C. The particulatehydrogel (2) had a temperature of 76° C. when it was introduced to adrier.

The particulate hydrogel (2) prepared as above had CRC of 28.3 [g/g],resin solid content of 51.8 wt %, water soluble content of 4.4 wt %,weight average molecular weight of the water soluble content of 258,606[Da], a weight average particle diameter (D50) of 676 μm, andlogarithmic standard deviation (σζ) of particle size distribution of0.87. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the particulate hydrogel (2).

The water absorbent resin particles (2) had a weight average particlediameter (D50) of 331 μm, logarithmic standard deviation (σζ) ofparticle size distribution of 0.32, CRC of 31.9 [g/g], and water solublecontent of 6.9 wt %, and included 150 μm passing particles (that is, theproportion of particles that would pass through a sieve having a meshsize of 150 μm) at 0.6 wt %. Table 3 shows physical properties of thewater absorbent resin powder (2).

Example 3

The present Example carried out, with respect to the belt-shapedhydrogel (1) prepared in Production Example 1, an operation identical tothat of Comparative Example 1 except that the gel was ground under theconditions that (i) the cutting length was 200 mm, (ii) no hot water orwater vapor was fed, and (iii) the number of revolutions of the screwshaft of the meat chopper was changed to 153 rpm. This operationprepared (i) particulate hydrogel (3), (ii) water absorbent resinparticles (3), and (iii) water absorbent resin powder (3). Example 3 hadgel grinding energy (GGE) of 31.9 [J/g] and gel grinding energy (2) (GGE(2)) of 19.2 [J/g]. The meat chopper had a current value of 115.8 Aduring the gel grinding. The hydrogel (1) had a temperature of 90° C.before the gel grinding, whereas after the gel grinding, the particulatehydrogel (3) had a temperature lowered to 87° C. The particulatehydrogel (3) had a temperature of 77° C. when it was introduced to adrier.

The particulate hydrogel (3) prepared as above had CRC of 28.3 [g/g],resin solid content of 51.2 wt %, water soluble content of 4.7 wt %,weight average molecular weight of the water soluble content of 267,785[Da], a weight average particle diameter (D50) of 705 μm, andlogarithmic standard deviation (σζ) of particle size distribution of0.85. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the particulate hydrogel (3).

The water absorbent resin particles (3) had a weight average particlediameter (D50) of 356 μm, logarithmic standard deviation (σζ) ofparticle size distribution of 0.34, CRC of 31.5 [g/g], and water solublecontent of 6.4 wt %, and included 150 μm passing particles (that is, theproportion of particles that would pass through a sieve having a meshsize of 150 μm) at 0.6 wt %. Table 3 shows physical properties of thewater absorbent resin powder (3).

Example 4

The present Example carried out, with respect to the belt-shapedhydrogel (1) prepared in Production Example 1, an operation identical tothat of Comparative Example 1 except that the gel was ground under thecondition that no hot water or water vapor was fed. This operationprepared (i) particulate hydrogel (4), (ii) water absorbent resinparticles (4), and (iii) water absorbent resin powder (4). Example 4 hadgel grinding energy (GGE) of 23.5 [J/g] and gel grinding energy (2) (GGE(2)) of 13.2 [J/g]. The meat chopper had a current value of 106.0 Aduring the gel grinding. The hydrogel (1) had a temperature of 90° C.before the gel grinding, whereas after the gel grinding, the particulatehydrogel (4) had a temperature lowered to 87° C. The particulatehydrogel (4) had a temperature of 77° C. when it was introduced to adrier.

The particulate hydrogel (4) prepared as above had CRC of 28.3 [g/g],resin solid content of 52.2 wt %, water soluble content of 4.7 wt %,weight average molecular weight of the water soluble content of 263,313[Da], a weight average particle diameter (D50) of 892 μm, andlogarithmic standard deviation (σζ) of particle size distribution of0.98. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the particulate hydrogel (4).

The water absorbent resin particles (4) had a weight average particlediameter (D50) of 351 μm, logarithmic standard deviation (σζ) ofparticle size distribution of 0.33, CRC of 31.6 [g/g], and water solublecontent of 6.4 wt %, and included 150 μm passing particles (that is, theproportion of particles that would pass through a sieve having a meshsize of 150 μm) at 0.5 wt %. Table 3 shows physical properties of thewater absorbent resin powder (4).

Example 5

The present Example fed the comparative particulate hydrogel (1),prepared in Comparative Example 1, to another screw extruder for anothergel grinding operation. The screw extruder was a meat chopper that (i)included, at an end, a porous die having a diameter of 68 mm, a porousdiameter of 11 mm, and a thickness of 8 mm and that (ii) had a screwshaft with a diameter of 21.0 mm. The present Example, in the statewhere the meat chopper was set so that the number of revolutions of thescrew shaft was 96 rpm, fed the comparative particulate hydrogel (1) at360 [g/min] to prepare particulate hydrogel (5). Example 5 fed no hotwater or water vapor during the another gel grinding operation. Thecomparative particulate hydrogel (1) had a temperature of 105° C. beforethe another gel grinding operation, whereas after the another gelgrinding operation, the particulate hydrogel (5) had a temperaturelowered to 95° C. The particulate hydrogel (5) had a temperature of 85°C. when it was introduced to a drier. Example 5 had gel grinding energy(GGE) of 34.3 [J/g] and gel grinding energy (2) (GGE (2)) of 18.3 [J/g].

The particulate hydrogel (5) prepared through the another gel grindingoperation had CRC of 28.5 [g/g], resin solid content of 49.1 wt %, watersoluble content of 4.4 wt %, weight average molecular weight of thewater soluble content of 269,885 [Da], a weight average particlediameter (D50) of 772 μm, and logarithmic standard deviation (σζ) ofparticle size distribution of 0.91. Table 1 shows the conditions appliedto the gel grinding step. Table 2 shows physical properties of theparticulate hydrogel (5).

The present Example next carried out, with respect to the particulatehydrogel (5) prepared as above, an operation (including such steps asdrying, grinding, classifying, and surface crosslinking) similar to thatof Comparative Example 1 to prepare water absorbent resin particles (5)and water absorbent resin powder (5).

The water absorbent resin particles (5) prepared through the aboveoperation had a weight average particle diameter (D50) of 360 μm,logarithmic standard deviation (σζ) of particle size distribution of0.33, CRC of 31.7 [g/g], and water soluble content of 7.3 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.6 wt%. Table 3 shows physical properties of the water absorbent resin powder(5).

Example 6

The present Example fed the comparative particulate hydrogel (1),prepared in Comparative Example 1, to another screw extruder for anothergel grinding operation. The screw extruder was a meat chopper that (i)included, at an end, a porous die having a diameter of 68 mm, a porousdiameter of 7.5 mm, and a thickness of 8 mm and that (ii) had a screwshaft with a diameter of 21.0 mm. The present Example, in the statewhere the meat chopper was set so that the number of revolutions of thescrew shaft was 172 rpm, fed the comparative particulate hydrogel (1) at360 [g/min] to prepare particulate hydrogel (6). Example 6 fed no hotwater or water vapor during the another gel grinding operation. Thecomparative particulate hydrogel (6) had a temperature of 105° C. beforethe another gel grinding operation, whereas after the another gelgrinding operation, the particulate hydrogel (6) had a temperaturelowered to 96° C. The particulate hydrogel (6) had a temperature of 86°C. when it was introduced to a drier. Example 6 had gel grinding energy(GGE) of 39.8 [J/g] and gel grinding energy (2) (GGE (2)) of 23.8 [J/g].

The particulate hydrogel (6) prepared through the another gel grindingoperation had CRC of 29.1 [g/g], resin solid content of 49.8 wt %, watersoluble content of 5.4 wt %, weight average molecular weight of thewater soluble content of 326,424 [Da], a weight average particlediameter (D50) of 367 μm, and logarithmic standard deviation (σζ) ofparticle size distribution of 0.71. Table 1 shows the conditions appliedto the gel grinding step. Table 2 shows physical properties of theparticulate hydrogel (6).

The present Example next carried out, with respect to the particulatehydrogel (6) prepared as above, an operation (including such steps asdrying, grinding, classifying, and surface crosslinking) similar to thatof Comparative Example 1 to prepare water absorbent resin particles (6)and water absorbent resin powder (6).

The water absorbent resin particles (6) prepared through the aboveoperation had a weight average particle diameter (D50) of 390 μm,logarithmic standard deviation (σζ) of particle size distribution of0.36, CRC of 32.5 [g/g], and water soluble content of 8.6 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.5 wt%. Table 3 shows physical properties of the water absorbent resin powder(6).

Example 7

The present Example fed the comparative particulate hydrogel (1),prepared in Comparative Example 1, to another screw extruder for anothergel grinding operation. The screw extruder was a meat chopper that (i)included, at an end, a porous die having a diameter of 68 mm, a porousdiameter of 7.5 mm, and a thickness of 8 mm and that (ii) had a screwshaft with a diameter of 21.0 mm. Example 7 repeated gel grinding whilechanging the porous diameter from 7.5 mm sequentially to 6.2 mm, 4.7 mm,and 3.2 mm The present Example, in the state where the meat chopper wasset so that the number of revolutions of the screw shaft was 172 rpm,fed the comparative particulate hydrogel (1) at 360 [g/min] to prepareparticulate hydrogel (7). Example 7 fed no hot water or water vaporduring the second and subsequent gel grinding operations. Example 7 hadgel grinding energy (GGE) of 72.5 [J/g] and gel grinding energy (2) (GGE(2)) of 36.1 [J/g].

The particulate hydrogel (7) prepared through the another gel grindingoperation had CRC of 29.5 [g/g], resin solid content of 50.3 wt %, watersoluble content of 6.3 wt %, weight average molecular weight of thewater soluble content of 553,670 [Da], a weight average particlediameter (D50) of 1990 μm, and logarithmic standard deviation (σζ) ofparticle size distribution of 0.94. Table 1 shows the conditions appliedto the gel grinding step. Table 2 shows physical properties of theparticulate hydrogel (7).

The present Example next carried out, with respect to the particulatehydrogel (7) prepared as above, an operation (including such steps asdrying, grinding, classifying, and surface crosslinking) similar to thatof Comparative Example 1 to prepare water absorbent resin particles (7)and water absorbent resin powder (7).

The water absorbent resin particles (7) prepared through the aboveoperation had a weight average particle diameter (D50) of 336 μm,logarithmic standard deviation (σζ) of particle size distribution of0.34, CRC of 32.2 [g/g], and water soluble content of 10.7 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.7 wt%. Table 3 shows physical properties of the water absorbent resin powder(7).

Production Example 2

The present Production Example carried out an operation identical tothat of Production Example 1 except that a composition of a monomeraqueous solution was changed as below. Namely, the present ProductionExample prepared a belt-shaped hydrogel (2) by carrying out an operationidentical to that of Production Example 1 except that the presentProduction Example prepared a monomer aqueous solution (2) containing(i) 193.3 parts by weight of acrylic acid, (ii) 163.03 parts by weightof a 48 wt % sodium hydroxide aqueous solution, (iii) 0.659 parts byweight of polyethyleneglycol diacrylate (average number n=9), (iv) 52parts by weight of a 0.1 wt % pentasodiumethylenediaminetetra(methylenephosphonate) aqueous solution, and (v) 134parts by weight of deionized water. The belt-shaped hydrogel (2) had CRCof 33.2 [g/g], resin solid content of 53.0 wt %, water soluble contentof 8.0 wt %, and weight average molecular weight of the water solublecontent of 468,684 [Da].

Comparative Example 2

The present Comparative Example prepared a comparative particulatehydrogel (2′) by carrying out, with respect to the belt-shaped hydrogel(2), gel grinding identical to that of Comparative Example 1.Thereafter, the present Comparative Example fed the comparativeparticulate hydrogel (2′) to another screw extruder to carry out anothergel grinding with respect to the comparative particulate hydrogel (2′).The another screw extruder was a meat chopper that (i) included, at anend, a porous die having a diameter of 68 mm, a porous diameter of 3.2mm, and a thickness of 8 mm that (ii) had a screw shaft with a diameterof 20.8 mm. The present Comparative Example, in the state where the meatchopper was set so that the number of revolutions of the screw shaft was172 rpm, fed 500 [g/min] of the comparative particulate hydrogel (2′) toprepare comparative particulate hydrogel (2). Note that the presentComparative Example fed neither water vapor nor hot water during theanother gel grinding. The present Comparative Example had gel grindingenergy (GGE) of 66.2 [J/g] and gel grinding energy (2) (GGE (2)) of 50.2[J/g].

The comparative particulate hydrogel (2) prepared through the anothergel grinding had CRC of 35.1 [g/g], resin solid content of 52.8 wt %,water soluble content of 15.2 wt %, and weight average molecular weightof the water soluble content of 1,091,000 [Da], a weight averageparticle diameter (D50) of 484 μm, and logarithmic standard deviation(σζ) of particle size distribution of 1.25. Table 1 shows the conditionsapplied to the gel grinding step. Table 2 shows physical properties ofthe comparative particulate hydrogel (2).

The present Comparative Example next carried out, with respect to thecomparative particulate hydrogel (2) prepared as above, an operation(including such steps as drying, grinding, classifying, and surfacecrosslinking) similar to that of Comparative Example 1 to preparecomparative water absorbent resin particles (2) and comparative waterabsorbent resin powder (2).

The comparative water absorbent resin particles (2) prepared through theabove operation had a weight average particle diameter (D50) of 392 μm,logarithmic standard deviation (σζ) of particle size distribution of0.36, CRC of 38.3 [g/g], water soluble content of 19.7 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.6 wt%. Note that Table 3 shows physical properties of the comparative waterabsorbent resin powder (2).

Production Example 3

The present Production Example produced water absorbent resin powder inconformity with Example 1, Example 2, and Comparative Example 1 ofJapanese Patent Application Publication, Tokukai, No. 2004-339502 A.Namely, the present Production Example fed, to a dispersing machine,5.83 [g/s] of a 48.5 wt % sodium hydroxide aqueous solution, 7.24 [g/s]of acrylic acid, 0.0287 [g/s] of a 30 wt % polyethyleneglycol diacrylateaqueous solution (average molecular weight: 487) as an internalcrosslinking agent, 3.32 [g/s] of deionized water, and 0.0893 [g/s] ofan aqueous solution (A) (a solution prepared by dissolving, in 97.4parts by weight of a 20 wt % acrylic acid aqueous solution, 0.989 partsby weight of 1-hydroxy-cyclohexyl-phenylketone and 1.08 parts by weightof a 45 wt % pentasodium diethylenetriamine pentaacetic acid aqueoussolution), so as to prepare a monomer aqueous solution (3). Note thatthe acrylic acid, the deionized water, the internal crosslinking agent,and the aqueous solution (A) were uniformed by a stirring machine andthen fed to the dispersing machine. The monomer aqueous solution (3)prepared as above was stable at a temperature of approximately 95° C.

Next, the present Production Example stirred the monomer aqueoussolution (3) by use of a static mixer in which an element was provided,the element having been obtained by adding, to a pipe having a pipediameter of 6 mm, a 1.5-fold twist having a length of 18.6 mm and adiameter of 6 mm. Then, the present Production Example added a 2 wt %sodium persulfate aqueous solution as a polymerization initiatorapproximately 3 cm downstream from an end part of the element at a flowrate of 0.151 [g/s]. Thereafter, the present Production Example carriedout polymerization by continuously feeding the resulting solution to abelt polymerization device provided with an endless belt (i) having alength of 3.8 m and a width of 60 cm and (ii) coated with fluorine. Thisprepared a belt-shaped hydrogel (3). Note that according to the beltpolymerization device, a UV lamp was provided in an upper part of thebelt, a bottom surface and a vicinity of the belt polymerization devicewere heated to approximately 100° C. and warmed, and a suction pipe forcollecting evaporated water was further provided at a central part ofthe polymerization device. Note also that after the addition of thepolymerization initiator, a pipe line had a length of 30 cm to an inletof the polymerization device. The belt-shaped hydrogel (3) prepared asabove had CRC of 32.5 [g/g], resin solid content of 58.0 wt %, watersoluble content of 5.2 wt %, and weight average molecular weight of thewater soluble content of 551,353 [Da].

Comparative Example 3

The present Comparative Example continuously produced water absorbentpolyacrylic acid (salt) resin powder subsequently to Production Example3.

Specifically, the present Comparative Example continuously fed, to ascrew extruder, the belt-shaped hydrogel (3) prepared in ProductionExample 3 and having a temperature of approximately 50° C., andsimultaneously ground the belt-shaped hydrogel (3) while injecting watervapor into the screw extruder via a water feed opening. The screwextruder used in the present Comparative Example was a screw extruderobtained by providing a meat chopper illustrated in FIG. 1 of JapanesePatent Application Publication, Tokukai, No. 2000-63527 A with a waterfeeding opening. When discharged from the meat chopper, comparativeparticulate hydrogel (3) prepared through the gel grinding operationgenerated steam and was free-flowing with a high flowability. Note thatthe hydrogel (3) which had not been subjected to gel grinding had atemperature of approximately 50° C. and the comparative particulatehydrogel (3) which had been subjected to gel grinding had a highertemperature of 55° C. Further, the comparative particulate hydrogel (3)had a temperature of 45° C. while being introduced into a drier.Comparative Example 3 had gel grinding energy (GGE) of 15.3 [J/g] andgel grinding energy (2) (GGE (2)) of 7.2 [J/g].

The comparative particulate hydrogel (3) had CRC of 32.4 [g/g], resinsolid content of 56.5 wt %, water soluble content of 5.5 wt %, andweight average molecular weight of the water soluble content of 555,210[Da], a weight average particle diameter (D50) of 2125 μm, andlogarithmic standard deviation (σζ) of particle size distribution of2.22. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the comparative particulatehydrogel (3).

The present Comparative Example next dried the comparative particulatehydrogel (3) with a hot-air drier at 180° C. for 35 minutes. Then, thepresent Comparative Example ground the comparative particulate hydrogel(3) (carried out the grinding step) by use of a roll mill. Thereafter,the present Comparative Example further classified the groundcomparative particulate hydrogel (3). This prepared comparative waterabsorbent resin particles (3) each having an irregularly broken shape.The hot air from the hot-air drier had an average wind velocity of 1.0[m/s] in a direction perpendicular to a plane on which the particulatehydrogel was placed. Note that the present Comparative Example measuredthe wind velocity of the hot air with use of constant-temperaturethermal anemometer Anemomaster 6162 produced by KANOMAX JAPANincorporated. The comparative water absorbent resin particles (3) had aweight average particle diameter (D50) of 351 μm, logarithmic standarddeviation (σζ) of particle size distribution of 0.34, CRC of 37.0 [g/g],and water soluble content of 12.0 wt %, and included 150 μm passingparticles (that is, the proportion of particles that would pass througha sieve having a mesh size 150 μm) at 3.1 wt %.

The present Comparative Example next (i) uniformly mixed, with 100 partsby weight of the comparative water absorbent resin particles (3), a(covalent bonding) surface crosslinking agent solution containing 0.3parts by weight of 1,4-butandiol, 0.6 parts by weight of propyleneglycol, and 3.0 parts by weight of deionized water, and (ii)heat-treated the resulting mixture at 208° C. for approximately 40minutes so that the resulting comparative water absorbent resin powder(3) would have CRC ranging from 26.6 [g/g] to 27.4 [g/g]. The presentComparative Example then cooled the resulting comparative waterabsorbent resin powder (3) and uniformly mixed with it a (ionic bonding)surface crosslinking agent solution containing 1.17 parts by weight of a27.5 wt % aluminum sulfate aqueous solution (8 wt % based on aluminumoxide), 0.196 parts by weight of a 60 wt % sodium lactate aqueoussolution, and 0.029 parts by weight of propylene glycol.

The present Comparative Example next crushed the resulting mixture (thatis, the granulating step) until it would pass through a JIS standardsieve having a mesh size of 710 μm. This prepared comparative waterabsorbent resin powder (3). Table 3 shows physical properties of thecomparative water absorbent resin powder (3).

Comparative Example 4

The present Comparative Example carried out, with respect to thebelt-shaped hydrogel (3) prepared in Production Example 3, an operationidentical to that of Comparative Example 3 except that water vapor wasreplaced with hot water having a temperature of 80° C. This operationprepared comparative particulate hydrogel (4), comparative waterabsorbent resin particles (4), and comparative water absorbent resinpowder (4). Note that the hydrogel (3) which had not been subjected togel grinding had a temperature of approximately 50° C. and thecomparative particulate hydrogel (4) which had been subjected to gelgrinding had a higher temperature of 52° C. Further, the comparativeparticulate hydrogel (4) had a temperature of 42° C. while beingintroduced into a drier. Comparative Example 4 had gel grinding energy(GGE) of 16.4 [J/g] and gel grinding energy (2) (GGE (2)) of 8.4 [J/g].

The comparative particulate hydrogel (4) prepared as above had CRC of32.5 [g/g], resin solid content of 55.0 wt %, water soluble content of5.5 wt %, and weight average molecular weight of the water solublecontent of 556,205 [Da], weight average particle diameter (D50) of 2304μm, and logarithmic standard deviation (σζ) of particle sizedistribution of 2.39. Table 1 shows the conditions applied to the gelgrinding step. Table 2 shows physical properties of the comparativeparticulate hydrogel (4).

The comparative water absorbent resin particles (4) had a weight averageparticle diameter (D50) of 350 μm, logarithmic standard deviation (σζ)of particle size distribution of 0.34, CRC of 37.0 [g/g], and watersoluble content of 12.0 wt %, and included 150 μm passing particles(that is, the proportion of particles that would pass through a sievehaving a mesh size 150 μm) at 2.7 wt %. Note that Table 3 shows physicalproperties of the comparative water absorbent resin powder (4).

Comparative Example 5

The present Comparative Example carried out, with respect to thebelt-shaped hydrogel (3) prepared in Production Example 3, an operationidentical to that of Comparative Example 3 except that the presentComparative Example carried out gel grinding without injecting anythinginto the screw extruder via the water feeding opening. This operationprepared comparative particulate hydrogel (5), comparative waterabsorbent resin particles (5), and comparative water absorbent resinpowder (5). Note that the comparative particulate hydrogel (5)discharged from the meat chopper was not completely in a line. Note alsothat the hydrogel (3) which had not been subjected to gel grinding had atemperature of approximately 50° C. and the comparative particulatehydrogel (5) which had been subjected to gel grinding had a lowertemperature of 45° C. Further, the comparative particulate hydrogel (5)had a temperature of 40° C. while being introduced into a drier.Comparative Example 5 had gel grinding energy (GGE) of 62.3 [J/g] andgel grinding energy (2) (GGE (2)) of 54.1 [J/g].

The comparative particulate hydrogel (5) prepared as above had CRC of33.1 [g/g], resin solid content of 58.0 wt %, water soluble content of13.1 wt %, and weight average molecular weight of the water solublecontent of 1, 087,542 [Da], a weight average particle diameter (D50) of1690 μm, and logarithmic standard deviation (σζ) of particle sizedistribution of 1.53. Table 1 shows the conditions applied to the gelgrinding step. Table 2 shows physical properties of the comparativeparticulate hydrogel (5).

The comparative water absorbent resin particles (5) had a weight averageparticle diameter (D50) of 347 μm, logarithmic standard deviation (σζ)of particle size distribution of 0.34, CRC of 36.0 [g/g], and watersoluble content of 21.0 wt %, and included 150 μm passing particles(that is, the proportion of particles that would pass through a sievehaving a mesh size 150 μm) at 3.5 wt %. Note that Table 3 shows physicalproperties of the comparative water absorbent resin powder (5).

Comparative Example 6

The present Comparative Example 6 selected, from the comparativeparticulate hydrogel (1) prepared in Comparative Example 1, thecomparative particulate hydrogel (1) having a particle diameter ofapproximately 2 mm, and regarded the selected comparative particulatehydrogel (1) as comparative particulate hydrogel (6). Note that thecomparative particulate hydrogel (6) had a temperature of 80° C. whilebeing introduced into a drier.

The comparative particulate hydrogel (6) had CRC of 28.0 [g/g], resinsolid content of 49.2 wt %, water soluble content of 4.3 wt %, andweight average molecular weight of the water soluble content of 220,518[Da], a weight average particle diameter (D50) of 2046 μm, andlogarithmic standard deviation (σζ) of particle size distribution of0.91. Table 1 shows the conditions applied to the gel grinding step.Table 2 shows physical properties of the comparative particulatehydrogel (6).

The present Comparative Example next carried out, with respect to thecomparative particulate hydrogel (6), an operation (including such stepsas drying, grinding, and classifying) similar to that of ComparativeExample 1 to prepare comparative water absorbent resin particles (6)each having an irregularly broken shape. The comparative water absorbentresin particles (6) had a weight average particle diameter (D50) of 398μm, logarithmic standard deviation (σζ) of particle size distribution of0.36, CRC of 32.5 [g/g], and water soluble content of 6.8 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.6 wt%.

The present Comparative Example next (i) uniformly mixed, with 100 partsby weight of the comparative water absorbent resin particles (6), a(covalent bonding) surface crosslinking agent solution containing 0.3parts by weight of 1,4-butandiol, 0.6 parts by weight of propyleneglycol, and 3.0 parts by weight of deionized water, and (ii)heat-treated the resulting mixture at 208° C. for approximately 40minutes so that the resulting comparative water absorbent resin powder(6) would have CRC ranging from 26.6 [g/g] to 27.4 [g/g]. The presentComparative Example then cooled the resulting comparative waterabsorbent resin powder (6) and uniformly mixed with it a (ionic bonding)surface crosslinking agent solution containing 1.17 parts by weight of a27.5 wt % aluminum sulfate aqueous solution (8 wt % based on aluminumoxide), 0.196 parts by weight of a 60 wt % sodium lactate aqueoussolution, and 0.029 parts by weight of propylene glycol.

The present Comparative Example next crushed the resulting mixture (thatis, the granulating step) until it would pass through a JIS standardsieve having a mesh size of 710 μm. This prepared comparative waterabsorbent resin powder (6). Table 3 shows physical properties of thecomparative water absorbent resin powder (6).

Example 8

The present Example next distributed (at this stage, the particulatehydrogel (1) had a temperature of 80° C.) the particulate hydrogel (1),prepared in Example 1, on a through-flow belt within 1 minute from theend of the gel grinding to dry the particulate hydrogel (1) at 185° C.for 30 minutes. This prepared a dried polymer (8). The through-flow belthad a moving rate of 1 [m/min]. The hot air had an average wind velocityof 0.5 [m/s] in a direction perpendicular to a direction in which thethrough-flow belt moved. Note that the present Example measured the windvelocity of the hot air with use of constant-temperature thermalanemometer Anemomaster 6162 produced by KANOMAX JAPAN incorporated.

The present Example next continuously fed the dried polymer (8),prepared through the drying step, to a three-stage roll mill to grind it(that is, the grinding step), and then classified the ground driedpolymer (8) with use of JIS standard sieves having respective mesh sizesof 710 μm and 175 μm. This prepared water absorbent resin particles (8)each having an irregularly broken shape. The water absorbent resinparticles (8) had a weight average particle diameter (D50) of 350 μm,logarithmic standard deviation (σζ) of particle size distribution of0.33, CRC of 28.9 [g/g], and water soluble content of 6.4 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.7 wt%.

The present Example next carried out, with respect to the waterabsorbent resin particles (8), a surface crosslinking treatment andgranulation identical to those carried out with respect to thecomparative water absorbent resin particles (6) of Comparative Example6. This prepared water absorbent resin powder (8). Table 3 showsphysical properties of the water absorbent resin powder (8).

Example 9

The present Example next distributed (at this stage, the particulatehydrogel (1) had a temperature of 80° C.) the particulate hydrogel (1),prepared in Example 1, on a through-flow belt within 1 minute from theend of the gel grinding to dry the particulate hydrogel (1) at 185° C.for 30 minutes. This prepared a dried polymer (9). The through-flow belthad a moving rate of 1 [m/min]. The hot air had an average wind velocityof 3.0 [m/s] in a direction perpendicular to a direction in which thethrough-flow belt moved. Note that the present Example measured the windvelocity of the hot air with use of constant-temperature thermalanemometer Anemomaster 6162 produced by KANOMAX JAPAN incorporated.

The present Example next continuously fed the dried polymer (9),prepared through the drying step, to a three-stage roll mill to grind it(that is, the grinding step), and then classified the ground driedpolymer (9) with use of JIS standard sieves having respective mesh sizesof 710 μm and 175 μm. This prepared water absorbent resin particles (9)each having an irregularly broken shape. The water absorbent resinparticles (9) had a weight average particle diameter (D50) of 330 μm,logarithmic standard deviation (σζ) of particle size distribution of0.35, CRC of 33.8 [g/g], and water soluble content of 7.9 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 1.5 wt%.

The present Example next carried out, with respect to the waterabsorbent resin particles (9), a surface crosslinking treatment andgranulation identical to those carried out with respect to thecomparative water absorbent resin particles (6) of Comparative Example6. This prepared water absorbent resin powder (9). Table 3 showsphysical properties of the water absorbent resin powder (9).

Comparative Example 7

The present Comparative Example next distributed (at this stage, theparticulate hydrogel (1) had a temperature of 80° C.) the particulatehydrogel (1), prepared in Example 1, on a through-flow belt within 1minute from the end of the gel grinding to dry the particulate hydrogel(1) at 130° C. for 30 minutes. This prepared a comparative dried polymer(7). The through-flow belt had a moving rate of 1 [m/min]. The hot airhad an average wind velocity of 1.0 [m/s] in a direction perpendicularto a direction in which the through-flow belt moved. Note that thepresent Example measured the wind velocity of the hot air with use ofconstant-temperature thermal anemometer Anemomaster 6162 produced byKANOMAX JAPAN incorporated.

The present Comparative Example next continuously fed the comparativedried polymer (7), prepared through the drying step, to a three-stageroll mill to grind it (that is, the grinding step), and then classifiedthe ground comparative dried polymer (7) with use of JIS standard sieveshaving respective mesh sizes of 710 μm and 175 μm. This preparedcomparative water absorbent resin particles (7) each having anirregularly broken shape. The comparative water absorbent resinparticles (7) had a weight average particle diameter (D50) of 370 μm,logarithmic standard deviation (σζ) of particle size distribution of0.34, CRC of 27.8 [g/g], and water soluble content of 6.4 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.7 wt%.

The present Example next carried out, with respect to the comparativewater absorbent resin particles (7), a surface crosslinking treatmentand granulation identical to those carried out with respect to thecomparative water absorbent resin particles (6) of Comparative Example6. This prepared comparative water absorbent resin powder (7). Table 3shows physical properties of the comparative water absorbent resinpowder (7).

Comparative Example 8

The present Comparative Example next distributed (at this stage, theparticulate hydrogel (1) had a temperature of 80° C.) the particulatehydrogel (1), prepared in Example 1, on a through-flow belt within 1minute from the end of the gel grinding to dry the particulate hydrogel(1) at 260° C. for 30 minutes. This prepared a comparative dried polymer(8). The through-flow belt had a moving rate of 1 [m/min]. The hot airhad an average wind velocity of 1.0 [m/s] in a direction perpendicularto a direction in which the through-flow belt moved. Note that thepresent Comparative Example measured the wind velocity of the hot airwith use of constant-temperature thermal anemometer Anemomaster 6162produced by KANOMAX JAPAN incorporated.

The present Comparative Example next continuously fed the comparativedried polymer (8), prepared through the drying step, to a three-stageroll mill to grind it (that is, the grinding step), and then classifiedthe ground comparative dried polymer (8) with use of JIS standard sieveshaving respective mesh sizes of 710 μm and 175 μm. This preparedcomparative water absorbent resin particles (8) each having anirregularly broken shape. The comparative water absorbent resinparticles (8) had a weight average particle diameter (D50) of 390 μm,logarithmic standard deviation (σζ) of particle size distribution of0.36, CRC of 32.0 [g/g], and water soluble content of 9.1 wt %, andincluded 150 μm passing particles (that is, the proportion of particlesthat would pass through a sieve having a mesh size of 150 μm) at 0.9 wt%.

The present Comparative Example next carried out, with respect to thecomparative water absorbent resin particles (8), a surface crosslinkingtreatment and granulation identical to those carried out with respect tothe comparative water absorbent resin particles (6) of ComparativeExample 6. This prepared comparative water absorbent resin powder (8).Table 3 shows physical properties of the comparative water absorbentresin powder (8).

Production Example 4

As in the case of Production Example 1, the present Production Exampleused a continuous production device to continuously produce waterabsorbent polyacrylic acid (salt) resin powder. Specifically, thepresent Production Example prepared a monomer aqueous solution (1)containing (i) 193.3 parts by weight of acrylic acid, (ii) 64.4 parts byweight of a 48 wt % sodium hydroxide aqueous solution, (iii) 1.26 partsby weight of polyethyleneglycol diacrylate (average number n=9), (iv) 52parts by weight of a 0.1 wt % pentasodium ethylenediaminetetra(methylene phosphonate) aqueous solution, and (v) 134 parts byweight of deionized water.

The present Production Example next continuously fed, with use of aconstant rate pump, the above monomer aqueous solution (1) with atemperature adjusted to 42° C., and then further continuously fed 97.1parts by weight of a 48 wt % sodium hydroxide aqueous solution for linemixing. Note that the temperature of the monomer aqueous solution (1)was at this stage raised to 87° C. due to heat of neutralization.

Further, the present Production Example next continuously fed 8.05 partsby weight of a 4 wt % sodium persulfate aqueous solution for linemixing, and then continuously fed the resulting mixture to a continuouspolymerization device, equipped with a planar polymerization beltprovided with a dam at each end, so that the fed mixture would have athickness of approximately 7.5 mm. The continuous polymerization devicecarried out polymerization continuously (polymerization period: 3minutes) to prepare a belt-shaped hydrogel (4). The belt-shaped hydrogel(4) had CRC of 27.7 [g/g], resin solid content of 53.3 wt %, watersoluble content of 3.8 wt %, and weight average molecular weight of thewater soluble content of 221,156 [Da].

Comparative Example 9

Subsequently to Production Example 4, the present Comparative Examplecarried out an operation identical to that of Comparative Example 1, andcontinuously produced water absorbent polyacrylic acid (salt) resinpowder. Table 1 shows the conditions applied to the gel grinding step.Table shows physical properties of comparative particulate hydrogel (9).Table 3 shows physical properties of comparative water absorbent resinpowder (9) thus prepared.

Comparative Examples 10 and 11

Comparative Examples 10 and 11 measured physical properties of waterabsorbent resin powder extracted from commercial disposable diapersdescribed in Comparative Examples 17 and 18 of an unpublished priorapplication No. PCT/JP2010/073254 (International Application Date: Dec.24, 2010).

Specifically, Comparative Example 10 measured an internal cell rate, CRC(absorbency without pressure), FSR (a water absorbing rate), andabsorption against pressure (AAP) of water absorbent resin extractedfrom a disposable diaper (produced by Unicharm Corporation: productname: “Mamy Poko (Registered Trademark)” purchased in Indonesia in July,2010 (referred to as comparative water absorbent resin powder (10)), andComparative Example 11 measured an internal cell rate, CRC (absorbencywithout pressure), FSR (a water absorbing rate), and absorption againstpressure (AAP) of water absorbent resin extracted from a disposablediaper (produced by dm: product name: “babylove aktiv plus” purchased inGermany in June, 2010 (referred to as comparative water absorbent resinpowder (11)). Table 3 shows results of the measurements.

Example 10

The present Example carried out an operation identical to that ofExample 3 except that the present Example replaced the (covalentbonding) surface crosslinking agent solution of Example 3 with asolution containing 0.5 parts by weight of ethylene carbonate and 3.0parts by weight of deionized water. This prepared water absorbent resinpowder (10). Table 3 shows physical properties of the water absorbentresin powder (10) thus prepared.

TABLE 1 Ext. MW up¹⁾ Gel Ext. up²⁾ GGE (2) in gel grinding in gelgrinding GGE [l/g] [l/g] step [Da] step [wt %] Comp. Ex. 1 17.9 8.77,297 0.3 Ex. 1 27.8 15.5 35,219 0.4 Ex. 2 28.2 15.8 40,229 0.4 Ex. 331.9 19.2 49,408 0.7 Ex. 4 23.5 13.2 44,936 0.7 Ex. 5 34.3 18.3 51,5080.4 Ex. 6 39.8 23.8 108,047 1.4 Ex. 7 72.5 36.1 335,293 2.3 Comp. Ex. 266.2 50.2 622,316 7.2 Comp. Ex. 3 15.3 7.2 3,857 0.3 Comp. Ex. 4 16.48.4 4,852 0.3 Comp. Ex. 5 62.3 54.1 536,189 7.9 Comp. Ex. 6 — — 2,1410.3 Comp. Ex. 9 17.2 8.1 6,503 0.2 ¹⁾Amount of weight average molecularweight of water soluble content to increase [Da] ²⁾Amount of gel Ext(water soluble content) to increase [wt %]

TABLE 2 Resin Water D50 of solid soluble particulate σζ of CRC contentcontent Ext. MW hydrogel³⁾ particulate [g/g] [wt %] [wt %] [Da] [μm]hydrogel⁴⁾ Comp. Comp. particulate 28.2 49.4 4.3 225,674 1041 1.74 Ex. 1hydrogel (1) Ex. 1 Particulate hydrogel (1) 28.3 50.8 4.4 253,596 7500.79 Ex. 2 Particulate hydrogel (2) 28.3 51.8 4.4 258,606 676 0.87 Ex. 3Particulate hydrogel (3) 28.3 51.2 4.7 267,785 705 0.85 Ex. 4Particulate hydrogel (4) 28.3 52.2 4.7 263,313 892 0.98 Ex. 5Particulate hydrogel (5) 28.5 49.1 4.4 269,885 772 0.91 Ex. 6Particulate hydrogel (6) 29.1 49.8 5.4 326,424 367 0.71 Ex. 7Particulate hydrogel (7) 29.5 50.3 6.3 553,670 1990 0.94 Comp. Comp.particulate 35.1 52.8 15.2 1,091,000 484 1.25 Ex. 2 hydrogel (2) Comp.Comp. particulate 32.4 56.5 5.5 555,210 2125 2.22 Ex. 3 hydrogel (3)Comp. Comp. particulate 32.5 55.0 5.5 556,205 2304 2.39 Ex. 4 hydrogel(4) Comp. Comp. particulate 33.1 58.0 13.1 1,087,542 1690 1.53 Ex. 5hydrogel (5) Comp. Comp. particulate 28.0 49.2 4.3 220,518 2046 0.91 Ex.6 hydrogel (6) Comp. Comp. particulate 27.9 50.1 4.0 227,659 1080 1.81Ex. 9 hydrogel (9) ³⁾Weight average particle diameter of particulatehydrogel [μm] ⁴⁾Logarithmic standard deviation of particle sizedistribution of particulate hydrogel

TABLE 3 Internal CRC AAP SFC FSR Cell Rate [g/g] [g/g] [5)] [g/g/s] [%]Comp. Comp. particulate resin powder (1) 27.4 23.1 69 0.29 2.4 Ex. 1 Ex.1 Particulate resin powder (1) 27.1 23.5 90 0.36 1.8 Ex. 2 Particulateresin powder (2) 27.0 23.8 99 0.37 1.1 Ex. 3 Particulate resin powder(3) 26.7 24.1 116 0.38 0.8 Ex. 4 Particulate resin powder (4) 27.0 23.380 0.32 2.3 Ex. 5 Particulate resin powder (5) 26.8 — 98 0.35 — Ex. 6Particulate resin powder (6) 27.0 — 100 0.38 — Ex. 7 Particulate resinpowder (7) 27.3 — 100 0.38 — Comp. Comp. particulate resin powder (2)27.1 — 30 0.29 — Ex. 2 Comp. Comp. particulate resin powder (3) 27.0 —47 0.23 — Ex. 3 Comp. Comp. particulate resin powder (4) 27.1 — 49 0.22— Ex. 4 Comp. Comp. particulate resin powder (5) 27.0 — 40 0.25 — Ex. 5Comp. Comp. particulate resin powder (6) 27.0 22.9 67 0.27 1.8 Ex. 6 Ex.8 Particulate resin powder (8) 27.1 19.1 56 0.33 1.5 Ex. 9 Particulateresin powder (9) 27.1 — 60 0.40 — Comp. Comp. particulate resin powder(7) 27.0 — 51 0.32 — Ex. 7 Comp. Comp. particulate resin powder (8) 26.9— 65 0.21 — Ex. 8 Comp. Comp. particulate resin powder (9) 27.4 22.8 690.31 2.6 Ex. 9 Comp. Comp. particulate resin powder (10) 35.0 11.1 00.48 1.2 Ex. 10 Comp. Comp. particulate resin powder (11) 31.8 22.0 110.24 0.6 Ex. 11 Ex. 10 Particulate resin powder (10) 26.9 24.0 110 0.380.8 5) [×10⁻⁷ · cm³ · s · g⁻¹]

Note that the prepared water absorbent resin powder was particulate (offine particles) as in the case of the absorbent resin particles whichhad not been subjected to surface crosslinking, and a particle size ofthe prepared water absorbent resin powder which had been subjected tosurface crosslinking was also substantially identical to or slightlygreater than that of the absorbent resin particles which had not beensubjected to surface crosslinking (not shown in the Tables).

CONCLUSION

As shown in Examples and Comparative Examples (described earlier), andTables 1 through 3, water absorbent resin powder in which a waterabsorbing rate (FSR) and permeability potential (SFC) are both attainedcan be produced by at least one of the gel grindings (1) through (4) ofthe present invention, i.e., (i) by carrying out gel grinding withrespect to hydrogel with gel grinding energy (GGE) of 18 [J/g] to 60[J/g] or with gel grinding energy (2) (GGE (2)) of 9 [J/g] to 40 [J/g],and thereafter drying the hydrogel and carrying out a surface treatmentwith respect to the hydrogel thus dried, (ii) by increasing weightaverage molecular weight of water soluble content of hydrogel by 10,000[Da] to 500,000 [Da], and thereafter drying the hydrogel and carryingout a surface treatment with respect to the hydrogel thus dried, or(iii) carrying out gel grinding with respect to hydrogel so thatparticulate hydrogel has a weight average particle diameter (D50) of 350μm to 2000 μm, logarithmic standard deviation (σζ) of particle sizedistribution of 0.2 to 1.0, and resin solid content of 10 wt % to 80 wt% and thereafter, under a specific condition, drying the particulatehydrogel and carrying out a surface treatment with respect to theparticulate hydrogel thus dried. Note that it may be preferable to carryout evaluation by use of gel grinding energy (2) (GGE (2)) since gelgrinding which is carried out a plurality of times in the gel grindingstep (e.g., the case of Example 7) increases idling energy.

As described earlier, according to the production method in accordancewith the present invention, permeability potential (SFC/more preferablein case of water absorbent resin having a smaller surface area) and awater absorbing rate (FSR/more preferable in case of water absorbentresin having a larger surface area) can be both attained. It has beendifficult to attain both the permeability potential and the waterabsorbing rate, which depend greatly on a surface area of waterabsorbent resin and conflict with each other. In particular, theproduction method of the present invention makes it possible to attainboth a high water absorbing rate such that FSR is not less than 0.30[g/g/s] and high permeability potential such that SFC is not less than70 [×10⁻⁷·cm³·s·g⁻¹]. The present invention can be suitably used for amethod for producing such water absorbent resin having high SFC and highFSR, which is suitable for, for example, sanitary supplies.

For example, Patent Literatures 1 through 50 described earlier have beenknown for improvement in, for example, a water absorbing rate andpermeability potential of water absorbent resin. In contrast, thepresent invention found that at least one of the specific gel grindings(1) through (4) allows improvement in water absorbing rate (e.g., FSR)and permeability potential (e.g., SFC) and attainment of both the waterabsorbing rate and the permeability potential.

INDUSTRIAL APPLICABILITY

Water absorbent resin powder produced by the production method of thepresent invention is useful for sanitary goods such as disposablediapers, sanitary napkins, and medical blood absorbers. The waterabsorbent resin powder is also applicable to a pet urine absorbingagent, an urine gelling agent for portable toilets, a freshnesspreserving agent for vegetables and fruits etc, a drip absorbing agentfor meat, and fish and shellfish, a cold insulator, disposable pocketwarmers, a gelling agent for battery, a water retaining agent forplants, soil, etc., a water condensation preventing agent, a water stopagent, a packing agent, artificial snow, etc.

REFERENCE SIGNS LIST

-   11: casing-   12: base-   13: screw-   14: feed opening-   15: hopper-   16: extrusion opening-   17: porous die-   18: rotational blade-   19: ring-   20: backflow preventing member-   20 a: stripe projection (backflow preventing member)-   21: motor-   22: linear projection

The invention claimed is:
 1. Water absorbent polyacrylic acid (salt)resin having: 95 wt % or more of particles whose diameter is not lessthan 150 μm and less than 850 μm; logarithmic standard deviation (σζ) ofparticle size distribution of 0.25 to 0.50; absorption against pressure(AAP) of 20 [g/g] or more; a water absorbing rate (FSR) of 0.30 [g/g/s]or more; a saline flow conductivity (SFC) of 20[×10⁻⁷·cm³·s·g⁻¹] ormore; and an internal cell ratio of greater than 1.0% to 2.5%, which iscalculated by {(real density)−(apparent density)}/(real density)×100,the water absorbent polyacrylic acid (salt) resin containing apolyvalent metal salt.
 2. The water absorbent polyacrylic acid (salt)resin as set forth in claim 1, further containing at least one of acationic polymer and inorganic fine particles.
 3. The water absorbentpolyacrylic acid (salt) resin as set forth in claim 1, wherein thesaline flow conductivity (SFC) is 50[×10⁻⁷·cm³·s·g⁻¹] or more.
 4. Thewater absorbent polyacrylic acid (salt) resin as set forth in claim 1,wherein a water-soluble content is 10 wt % or less.
 5. The waterabsorbent polyacrylic acid (salt) resin as set forth in claim 1, whereinbulk specific gravity is in a range of 0.50[g/cm³] to 0.80[g/cm³]. 6.The water absorbent polyacrylic acid (salt) resin as set forth in claim1, being crosslinked by combination use of an ionic bonding surfacecrosslinking agent and a covalent bonding surface crosslinking agent. 7.The water absorbent polyacrylic acid (salt) resin as set forth in claim1, wherein the polyvalent metal salt is aluminum or zirconium.